diff --git a/.travis.yml b/.travis.yml
index 5309d487..51256dcf 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,13 +4,15 @@ sudo: false
 dist: trusty
 
 php:
-  - 5.5
-  - 5.6
-  - 7.0
   - 7.1
+  - 7.2
+  - 7.3
+
+before_install:
+  - composer global require phpunit/phpunit ^7.0
 
 before_script:
   - composer update --dev
 
 script:
-  - phpunit --configuration phpunit.travis.xml
+  - /home/travis/.config/composer/vendor/bin/phpunit --configuration phpunit.travis.xml
diff --git a/Mbstring.php b/Mbstring.php
index a3c87fa3..6e48ede2 100644
--- a/Mbstring.php
+++ b/Mbstring.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -617,7 +617,7 @@ public static function isUtf8($str)
         $len = strlen($str);
 
         for ($i = 0; $i < $len; $i++) {
-            $in = ord($str{$i});
+            $in = ord($str[$i]);
 
             if ($mState === 0) {
                 // When mState is zero we expect either a US-ASCII character or a
diff --git a/SimpleTemplate.php b/SimpleTemplate.php
index 22667bd0..4d98a4ad 100644
--- a/SimpleTemplate.php
+++ b/SimpleTemplate.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -49,7 +49,7 @@ function ($match) use ($data) {
                     return $return;
                 }
             },
-            $string
+            (string) $string
         );
     }
 }
diff --git a/Str.php b/Str.php
index 5bfe6a06..d937aa50 100644
--- a/Str.php
+++ b/Str.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of ww4 project.
  *
@@ -17,15 +17,15 @@
  */
 class Str
 {
-    const CASE_SENSITIVE = true;
+    public const CASE_SENSITIVE = true;
 
-    const CASE_INSENSITIVE = false;
+    public const CASE_INSENSITIVE = false;
 
-    const ENCODING_DEFAULT_ISO = 'ISO-8859-1';
+    public const ENCODING_DEFAULT_ISO = 'ISO-8859-1';
 
-    const ENCODING_UTF8 = 'UTF-8';
+    public const ENCODING_UTF8 = 'UTF-8';
 
-    const ENCODING_US_ASCII = 'US-ASCII';
+    public const ENCODING_US_ASCII = 'US-ASCII';
 
     /**
      * Convert all to string.
@@ -35,10 +35,18 @@ class Str
      *
      * @return  string
      *
-     * @since  __DEPLOY_VERSION__
+     * @since  3.5
      */
     public static function toString($data, bool $dump = true): string
     {
+        if (is_callable($data)) {
+            return static::toString($data());
+        }
+
+        if (is_stringable($data)) {
+            return (string) $data;
+        }
+
         if (is_array($data)) {
             $data = $dump ? Arr::dump($data) : 'Array()';
         }
diff --git a/StrNormalise.php b/StrNormalise.php
new file mode 100644
index 00000000..b140241b
--- /dev/null
+++ b/StrNormalise.php
@@ -0,0 +1,226 @@
+<?php
+
+/**
+ * Part of ww4 project.
+ *
+ * @copyright  Copyright (C) 2019 __ORGANIZATION__.
+ * @license    LGPL-2.0-or-later
+ */
+
+declare(strict_types=1);
+
+namespace Windwalker\String;
+
+use Windwalker\Utilities\Arr;
+
+/**
+ * The StrNotmalise class.
+ *
+ * @since  3.5.17
+ */
+class StrNormalise
+{
+    public static function splitCamelCase(string $str): array
+    {
+        return preg_split('/(?<=[^A-Z_])(?=[A-Z])|(?<=[A-Z])(?=[A-Z][^A-Z_])/x', $str);
+    }
+
+    public static function splitBy(string $str, string $separator = '_'): array
+    {
+        return array_filter(explode($separator, $str), 'strlen');
+    }
+
+    public static function splitAll(string $str): array
+    {
+        $strs = preg_split('#[ \-_\.]+#', $str);
+
+        $strs = array_map([static::class, 'splitCamelCase'], $strs);
+
+        return array_values(Arr::flatten($strs));
+    }
+
+    /**
+     * Separate a string by custom separator.
+     *
+     * @param  string  $input      The string input (ASCII only).
+     * @param  string  $separator  The separator to want to separate it.
+     *
+     * @return  string  The string be converted.
+     *
+     * @since   2.1
+     */
+    public static function separate(string $input, string $separator = '_'): string
+    {
+        return implode($separator, self::splitAll($input));
+    }
+
+    /**
+     * Method to convert a string into camel case.
+     *
+     * @param  string  $input  The string input (ASCII only).
+     *
+     * @return  string  The camel case string.
+     */
+    public static function toCamelCase(string $input): string
+    {
+        return lcfirst(static::toPascalCase($input));
+    }
+
+    /**
+     * Method to convert a string into pascal case.
+     *
+     * @param  string  $input  The string input (ASCII only).
+     *
+     * @return  string  The camel case string.
+     *
+     * @since   2.0
+     */
+    public static function toPascalCase(string $input): string
+    {
+        return implode('', array_map('ucfirst', self::splitAll($input)));
+    }
+
+    /**
+     * Method to convert a string into dash separated form.
+     *
+     * @param  string  $input  The string input (ASCII only).
+     *
+     * @return  string  The dash separated string.
+     *
+     * @since   2.0
+     */
+    public static function toDashSeparated(string $input): string
+    {
+        return self::separate($input, '-');
+    }
+
+    /**
+     * Dash separated alias.
+     *
+     * @param  string  $input
+     *
+     * @return  string
+     */
+    public static function toKebabCase(string $input): string
+    {
+        return static::toDashSeparated($input);
+    }
+
+    /**
+     * Method to convert a string into space separated form.
+     *
+     * @param  string  $input  The string input (ASCII only).
+     *
+     * @return  string  The space separated string.
+     *
+     * @since   2.0
+     */
+    public static function toSpaceSeparated(string $input): string
+    {
+        // Convert underscores and dashes to spaces.
+        return static::separate($input, ' ');
+    }
+
+    /**
+     * Method to convert a string into dot separated form.
+     *
+     * @param  string  $input  The string input (ASCII only).
+     *
+     * @return  string  The dot separated string.
+     *
+     * @since   2.1
+     */
+    public static function toDotSeparated(string $input): string
+    {
+        // Convert underscores and dashes to dots.
+        return static::separate($input, '.');
+    }
+
+    /**
+     * Method to convert a string into underscore separated form.
+     *
+     * @param  string  $input  The string input (ASCII only).
+     *
+     * @return  string  The underscore separated string.
+     *
+     * @since   2.0
+     */
+    public static function toSnakeCase(string $input): string
+    {
+        // Convert spaces and dashes to underscores.
+        return static::separate($input, '_');
+    }
+
+    /**
+     * Alias of toSnakeCase()
+     *
+     * @param  string  $input
+     *
+     * @return  string
+     */
+    public static function toUnderscoreSeparated(string $input): string
+    {
+        return static::toSnakeCase($input);
+    }
+
+    /**
+     * Method to convert a string into variable form.
+     *
+     * @param  string  $input  The string input (ASCII only).
+     *
+     * @return  string  The variable string.
+     *
+     * @since   2.0
+     */
+    public static function toVariable(string $input): string
+    {
+        // Remove dashes and underscores, then convert to camel case.
+        $input = self::toCamelCase($input);
+
+        // Remove leading digits.
+        $input = preg_replace('#^[0-9]+.*$#', '', $input);
+
+        // Lowercase the first character.
+        $input[0] = strtolower($input[0] ?? '');
+
+        return $input;
+    }
+
+    /**
+     * Method to convert a string into key form.
+     *
+     * @param  string  $input  The string input (ASCII only).
+     *
+     * @return  string  The key string.
+     *
+     * @since   2.0
+     */
+    public static function toKey(string $input): string
+    {
+        // Remove spaces and dashes, then convert to lower case.
+        $input = self::toUnderscoreSeparated($input);
+        $input = strtolower($input);
+
+        return $input;
+    }
+
+    /**
+     * Convert to standard PSR-0/PSR-4 class name.
+     *
+     * @param  string  $class  The class name string.
+     *
+     * @return  string Normalised class name.
+     *
+     * @since   2.0
+     */
+    public static function toClassNamespace(string $class): string
+    {
+        $class = trim($class, '\\');
+
+        $class = (string) str_replace(['\\', '/'], ' ', $class);
+
+        $class = ucwords($class);
+
+        return str_replace(' ', '\\', $class);
+    }
+}
diff --git a/StringHelper.php b/StringHelper.php
index 59abd196..786516d9 100644
--- a/StringHelper.php
+++ b/StringHelper.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/StringInflector.php b/StringInflector.php
index cb647182..7fe91462 100644
--- a/StringInflector.php
+++ b/StringInflector.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -193,7 +193,7 @@ private function matchRegexRule($word, $ruleType)
         // Cycle through the regex rules.
         foreach ($this->rules[$ruleType] as $regex => $replacement) {
             $matches = 0;
-            $matchedWord = preg_replace($regex, $replacement, $word, -1, $matches);
+            $matchedWord = preg_replace($regex, $replacement, (string) $word, -1, $matches);
 
             if ($matches > 0) {
                 return $matchedWord;
diff --git a/StringNormalise.php b/StringNormalise.php
index 334ccedb..268db4c4 100644
--- a/StringNormalise.php
+++ b/StringNormalise.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -14,6 +14,8 @@
  * This class is based on Joomla String package
  *
  * @since  2.0
+ *
+ * @deprecated Use {@see \Windwalker\String\StrNormalise} instead.
  */
 abstract class StringNormalise
 {
diff --git a/StringObject.php b/StringObject.php
index d4cd4ad3..e6958414 100644
--- a/StringObject.php
+++ b/StringObject.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -194,6 +194,7 @@ protected function callProxy($class, $method, array $args)
      *        <b>Traversable</b>
      * @since 5.0.0
      */
+    #[\ReturnTypeWillChange]
     public function getIterator()
     {
         return new \ArrayIterator($this->chop());
@@ -214,6 +215,7 @@ public function getIterator()
      * The return value will be casted to boolean if non-boolean was returned.
      * @since 5.0.0
      */
+    #[\ReturnTypeWillChange]
     public function offsetExists($offset)
     {
         $offset = $offset >= 0 ? $offset : (int) abs($offset) - 1;
@@ -228,6 +230,7 @@ public function offsetExists($offset)
      *
      * @return string Can return all value types.
      */
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         return $this->getChar($offset);
@@ -248,6 +251,7 @@ public function offsetGet($offset)
      * @return void
      * @since 5.0.0
      */
+    #[\ReturnTypeWillChange]
     public function offsetSet($offset, $string)
     {
         $this->string = Mbstring::substrReplace($this->string, $string, $offset, 1, $this->encoding);
@@ -265,6 +269,7 @@ public function offsetSet($offset, $string)
      * @return void
      * @since 5.0.0
      */
+    #[\ReturnTypeWillChange]
     public function offsetUnset($offset)
     {
         if ($this->length() < abs($offset)) {
@@ -284,6 +289,7 @@ public function offsetUnset($offset)
      *        The return value is cast to an integer.
      * @since 5.1.0
      */
+    #[\ReturnTypeWillChange]
     public function count()
     {
         return $this->length();
@@ -605,11 +611,8 @@ public function indexOfLast($search)
      */
     public function explode($delimiter, $limit = null)
     {
-        // Fix HHVM default explode limit issue
-        // @see  https://github.com/facebook/hhvm/issues/7696
-        // @see  https://3v4l.org/fllad
         if ($limit === null) {
-            $limit = defined('HHVM_VERSION') ? 0x7FFFFFFF : PHP_INT_MAX;
+            return explode($delimiter, $this->string);
         }
 
         return explode($delimiter, $this->string, $limit);
@@ -630,4 +633,34 @@ function ($new) use ($callback) {
             }
         );
     }
+
+    /**
+     * pipe
+     *
+     * @param  callable  $callback
+     *
+     * @return  static
+     *
+     * @since  3.5.14
+     */
+    public function pipe(callable $callback): self
+    {
+        return $callback($this);
+    }
+
+    /**
+     * clearHtml
+     *
+     * @param string|null $allowTags
+     *
+     * @return  StringObject
+     *
+     * @since  3.5.13
+     */
+    public function stripHtmlTags(?string $allowTags = null)
+    {
+        return $this->cloneInstance(function (self $new) use ($allowTags) {
+            $new->string = strip_tags($new->string, $allowTags);
+        });
+    }
 }
diff --git a/Test/InflectorTest.php b/Test/InflectorTest.php
index 73e59445..7ed104e0 100644
--- a/Test/InflectorTest.php
+++ b/Test/InflectorTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * @copyright  Copyright (C) 2019 LYRASOFT Source Matters, Inc.
  * @license    LGPL-2.0-or-later
@@ -89,7 +89,7 @@ public function seedSinglePlural()
      *
      * @since   2.0
      */
-    protected function setUp()
+    protected function setUp(): void
     {
         parent::setUp();
 
@@ -154,11 +154,12 @@ public function testAddRule()
      *
      * @throws \ReflectionException
      * @covers  \Windwalker\String\StringInflector::addRule
-     * @expectedException  InvalidArgumentException
      * @since   2.0
      */
     public function testAddRuleException()
     {
+        $this->expectException(\InvalidArgumentException::class);
+
         TestHelper::invoke($this->StringInflector, 'addRule', new \stdClass(), 'singular');
     }
 
diff --git a/Test/MbstringTest.php b/Test/MbstringTest.php
index 8924b879..d55bd258 100644
--- a/Test/MbstringTest.php
+++ b/Test/MbstringTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project Test files.  @codingStandardsIgnoreStart
  *
diff --git a/Test/NormaliseTest.php b/Test/NormaliseTest.php
index 4628c697..1740cd11 100644
--- a/Test/NormaliseTest.php
+++ b/Test/NormaliseTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * @copyright  Copyright (C) 2019 LYRASOFT Source Matters, Inc.
  * @license    LGPL-2.0-or-later
@@ -6,7 +6,7 @@
 
 // phpcs:disable
 
-namespace Windwalker\String\Tests;
+namespace Windwalker\String\Test;
 
 use Windwalker\String\StringNormalise;
 
diff --git a/Test/SimpleTemplateTest.php b/Test/SimpleTemplateTest.php
index df5a23ec..83b4042a 100644
--- a/Test/SimpleTemplateTest.php
+++ b/Test/SimpleTemplateTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project Test files.  @codingStandardsIgnoreStart
  *
@@ -23,7 +23,7 @@ class SimpleTemplateTest extends \PHPUnit\Framework\TestCase
      *
      * @return void
      */
-    protected function tearDown()
+    protected function tearDown(): void
     {
     }
 
diff --git a/Test/StrTest.php b/Test/StrTest.php
index 05d2e7ed..3f8816d8 100644
--- a/Test/StrTest.php
+++ b/Test/StrTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of ww4 project.
  *
diff --git a/Test/StringHelperTest.php b/Test/StringHelperTest.php
index 39f2ec76..7fc788b0 100644
--- a/Test/StringHelperTest.php
+++ b/Test/StringHelperTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project Test files.  @codingStandardsIgnoreStart
  *
diff --git a/Test/StringObjectTest.php b/Test/StringObjectTest.php
index 1eb1c16d..0448bad7 100644
--- a/Test/StringObjectTest.php
+++ b/Test/StringObjectTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of ww4 project.
  *
@@ -29,7 +29,7 @@ class StringObjectTest extends TestCase
      *
      * @return void
      */
-    protected function tearDown()
+    protected function tearDown(): void
     {
     }
 
@@ -40,7 +40,7 @@ protected function tearDown()
      */
     public function testFunctionCreate()
     {
-        $s = str('白日依山盡', StringObject::ENCODING_US_ASCII);
+        $s = \Windwalker\str('白日依山盡', StringObject::ENCODING_US_ASCII);
 
         self::assertInstanceOf(StringObject::class, $s);
         self::assertEquals('白日依山盡', $s->getString());
@@ -158,6 +158,7 @@ public function testOffsetGet($offset, $expected)
      *
      * @return  array
      */
+    #[\ReturnTypeWillChange]
     public function offsetGetProvider()
     {
         return [
@@ -196,6 +197,7 @@ public function testOffsetSet($string, $replace, $offset, $expected)
      *
      * @return  array
      */
+    #[\ReturnTypeWillChange]
     public function offsetSetProvider()
     {
         return [
@@ -230,6 +232,7 @@ public function testOffsetUnset($string, $offset, $expected)
      *
      * @return  array
      */
+    #[\ReturnTypeWillChange]
     public function offsetUnsetProvider()
     {
         return [
diff --git a/Test/Utf8StringTest.php b/Test/Utf8StringTest.php
index 63fdd6ce..d5573ce7 100644
--- a/Test/Utf8StringTest.php
+++ b/Test/Utf8StringTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project Test files.  @codingStandardsIgnoreStart
  *
@@ -29,7 +29,7 @@ class Utf8StringTest extends \PHPUnit\Framework\TestCase
      *
      * @return void
      */
-    protected function setUp()
+    protected function setUp(): void
     {
     }
 
@@ -39,7 +39,7 @@ protected function setUp()
      *
      * @return void
      */
-    protected function tearDown()
+    protected function tearDown(): void
     {
     }
 
diff --git a/Utf8String.php b/Utf8String.php
index 847c7cb9..954355ab 100644
--- a/Utf8String.php
+++ b/Utf8String.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/composer.json b/composer.json
index af004826..bce2d9b8 100644
--- a/composer.json
+++ b/composer.json
@@ -6,11 +6,11 @@
     "homepage": "https://github.com/ventoviro/windwalker-string",
     "license": "LGPL-2.0-or-later",
     "require": {
-        "php": ">=5.5.9"
+        "php": ">=7.1.3"
     },
     "require-dev": {
         "windwalker/test": "~3.0",
-        "windwalker/utilities": "~3.0"
+        "windwalker/utilities": "dev-master"
     },
     "suggest": {
         "windwalker/utilities": "Install ~3.0 if you require SimpleTemplate template engine."
diff --git a/functions.php b/functions.php
index 288d3852..2c4ea37a 100644
--- a/functions.php
+++ b/functions.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of ww4 project.
  *
@@ -6,19 +6,42 @@
  * @license    Please see LICENSE file.
  */
 
-use Windwalker\String\StringObject;
+namespace {
 
-if (!function_exists('str')) {
-    /**
-     * str
-     *
-     * @param string      $string
-     * @param null|string $encoding
-     *
-     * @return  StringObject
-     */
-    function str($string = '', $encoding = StringObject::ENCODING_UTF8)
-    {
-        return new StringObject($string, $encoding);
+    use Windwalker\String\StringObject;
+
+    if (!function_exists('str')) {
+        /**
+         * str
+         *
+         * @param string      $string
+         * @param null|string $encoding
+         *
+         * @return  StringObject
+         */
+        function str($string = '', $encoding = StringObject::ENCODING_UTF8)
+        {
+            return new StringObject($string, $encoding);
+        }
+    }
+}
+
+namespace Windwalker {
+
+    use Windwalker\String\StringObject;
+
+    if (!function_exists('\Windwalker\str')) {
+        /**
+         * str
+         *
+         * @param string      $string
+         * @param null|string $encoding
+         *
+         * @return  StringObject
+         */
+        function str($string = '', $encoding = StringObject::ENCODING_UTF8)
+        {
+            return new StringObject($string, $encoding);
+        }
     }
 }
diff --git a/phputf8/mbstring/core.php b/phputf8/mbstring/core.php
index e836aee7..4dea993b 100644
--- a/phputf8/mbstring/core.php
+++ b/phputf8/mbstring/core.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -105,9 +105,9 @@ function utf8_strrpos($str, $search, $offset = false)
 function utf8_substr($str, $offset, $length = false)
 {
     if ($length === false) {
-        return mb_substr($str, $offset);
+        return mb_substr($str, (int) $offset);
     } else {
-        return mb_substr($str, $offset, $length);
+        return mb_substr($str, (int) $offset, (int) $length);
     }
 }
 //--------------------------------------------------------------------
@@ -127,7 +127,7 @@ function utf8_substr($str, $offset, $length = false)
  */
 function utf8_strtolower($str)
 {
-    return mb_strtolower($str);
+    return mb_strtolower((string) $str);
 }
 
 //--------------------------------------------------------------------
diff --git a/phputf8/native/core.php b/phputf8/native/core.php
index a077b5e2..b6c4b38f 100644
--- a/phputf8/native/core.php
+++ b/phputf8/native/core.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/ord.php b/phputf8/ord.php
index 1a2603c1..aa06d053 100644
--- a/phputf8/ord.php
+++ b/phputf8/ord.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -27,36 +27,36 @@ function utf8_ord($chr)
         return $ord0;
     }
 
-    if (!isset($chr{1})) {
+    if (!isset($chr[1])) {
         trigger_error('Short sequence - at least 2 bytes expected, only 1 seen');
 
         return false;
     }
 
-    $ord1 = ord($chr{1});
+    $ord1 = ord($chr[1]);
     if ($ord0 >= 192 && $ord0 <= 223) {
         return ($ord0 - 192) * 64
             + ($ord1 - 128);
     }
 
-    if (!isset($chr{2})) {
+    if (!isset($chr[2])) {
         trigger_error('Short sequence - at least 3 bytes expected, only 2 seen');
 
         return false;
     }
-    $ord2 = ord($chr{2});
+    $ord2 = ord($chr[2]);
     if ($ord0 >= 224 && $ord0 <= 239) {
         return ($ord0 - 224) * 4096
             + ($ord1 - 128) * 64
             + ($ord2 - 128);
     }
 
-    if (!isset($chr{3})) {
+    if (!isset($chr[3])) {
         trigger_error('Short sequence - at least 4 bytes expected, only 3 seen');
 
         return false;
     }
-    $ord3 = ord($chr{3});
+    $ord3 = ord($chr[3]);
     if ($ord0 >= 240 && $ord0 <= 247) {
         return ($ord0 - 240) * 262144
             + ($ord1 - 128) * 4096
@@ -64,12 +64,12 @@ function utf8_ord($chr)
             + ($ord3 - 128);
     }
 
-    if (!isset($chr{4})) {
+    if (!isset($chr[4])) {
         trigger_error('Short sequence - at least 5 bytes expected, only 4 seen');
 
         return false;
     }
-    $ord4 = ord($chr{4});
+    $ord4 = ord($chr[4]);
     if ($ord0 >= 248 && $ord0 <= 251) {
         return ($ord0 - 248) * 16777216
             + ($ord1 - 128) * 262144
@@ -78,7 +78,7 @@ function utf8_ord($chr)
             + ($ord4 - 128);
     }
 
-    if (!isset($chr{5})) {
+    if (!isset($chr[5])) {
         trigger_error('Short sequence - at least 6 bytes expected, only 5 seen');
 
         return false;
@@ -89,7 +89,7 @@ function utf8_ord($chr)
             + ($ord2 - 128) * 262144
             + ($ord3 - 128) * 4096
             + ($ord4 - 128) * 64
-            + (ord($chr{5}) - 128);
+            + (ord($chr[5]) - 128);
     }
 
     if ($ord0 >= 254 && $ord0 <= 255) {
diff --git a/phputf8/str_ireplace.php b/phputf8/str_ireplace.php
index e449b247..bcdb8d80 100644
--- a/phputf8/str_ireplace.php
+++ b/phputf8/str_ireplace.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/str_pad.php b/phputf8/str_pad.php
index 976fae71..915823e2 100644
--- a/phputf8/str_pad.php
+++ b/phputf8/str_pad.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/str_split.php b/phputf8/str_split.php
index 9e4da516..22023b0f 100644
--- a/phputf8/str_split.php
+++ b/phputf8/str_split.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -24,7 +24,7 @@
  */
 function utf8_str_split($str, $split_len = 1)
 {
-    if (!preg_match('/^[0-9]+$/', $split_len) || $split_len < 1) {
+    if (!preg_match('/^[0-9]+$/', (string) $split_len) || $split_len < 1) {
         return false;
     }
 
diff --git a/phputf8/strcasecmp.php b/phputf8/strcasecmp.php
index e52b7877..c7234c52 100644
--- a/phputf8/strcasecmp.php
+++ b/phputf8/strcasecmp.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/strcspn.php b/phputf8/strcspn.php
index dc6e5e4d..c54e7054 100644
--- a/phputf8/strcspn.php
+++ b/phputf8/strcspn.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/stristr.php b/phputf8/stristr.php
index 113fd8a5..c3b23db0 100644
--- a/phputf8/stristr.php
+++ b/phputf8/stristr.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/strrev.php b/phputf8/strrev.php
index 2b3ab714..458da870 100644
--- a/phputf8/strrev.php
+++ b/phputf8/strrev.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/strspn.php b/phputf8/strspn.php
index 8a241ac1..2beee68f 100644
--- a/phputf8/strspn.php
+++ b/phputf8/strspn.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/substr_replace.php b/phputf8/substr_replace.php
index e830a5d3..da3a49dc 100644
--- a/phputf8/substr_replace.php
+++ b/phputf8/substr_replace.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/trim.php b/phputf8/trim.php
index c1bcd123..427fec2d 100644
--- a/phputf8/trim.php
+++ b/phputf8/trim.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/ucfirst.php b/phputf8/ucfirst.php
index d87a579d..72902343 100644
--- a/phputf8/ucfirst.php
+++ b/phputf8/ucfirst.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/ucwords.php b/phputf8/ucwords.php
index 0fdb2e8d..8c2dae4d 100644
--- a/phputf8/ucwords.php
+++ b/phputf8/ucwords.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/utf8.php b/phputf8/utf8.php
index 64fc3966..7303de82 100644
--- a/phputf8/utf8.php
+++ b/phputf8/utf8.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -23,7 +23,7 @@
  * encoding
  */
 if (extension_loaded('mbstring')) {
-    if (ini_get('mbstring.func_overload') & MB_OVERLOAD_STRING) {
+    if (defined('MB_OVERLOAD_STRING') && ini_get('mbstring.func_overload') & MB_OVERLOAD_STRING) {
         trigger_error('String functions are overloaded by mbstring', E_USER_ERROR);
     }
     mb_internal_encoding('UTF-8');
diff --git a/phputf8/utils/ascii.php b/phputf8/utils/ascii.php
index ed2ffe85..106eaaad 100644
--- a/phputf8/utils/ascii.php
+++ b/phputf8/utils/ascii.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.  @codingStandardsIgnoreStart
  *
diff --git a/phputf8/utils/bad.php b/phputf8/utils/bad.php
index f809b053..dcece329 100644
--- a/phputf8/utils/bad.php
+++ b/phputf8/utils/bad.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -254,7 +254,7 @@ function utf8_bad_identify($str, &$i)
     $len = strlen($str);
 
     for ($i = 0; $i < $len; $i++) {
-        $in = ord($str{$i});
+        $in = ord($str[$i]);
 
         if ($mState == 0) {
             // When mState is zero we expect either a US-ASCII character or a
diff --git a/phputf8/utils/patterns.php b/phputf8/utils/patterns.php
index fa18dd58..f97274c0 100644
--- a/phputf8/utils/patterns.php
+++ b/phputf8/utils/patterns.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/utils/position.php b/phputf8/utils/position.php
index 38aabee3..964fc267 100644
--- a/phputf8/utils/position.php
+++ b/phputf8/utils/position.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/utils/specials.php b/phputf8/utils/specials.php
index 16ac41da..216838d5 100644
--- a/phputf8/utils/specials.php
+++ b/phputf8/utils/specials.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
diff --git a/phputf8/utils/unicode.php b/phputf8/utils/unicode.php
index 868342bb..fc2611ea 100644
--- a/phputf8/utils/unicode.php
+++ b/phputf8/utils/unicode.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -39,7 +39,7 @@ function utf8_to_unicode($str)
     $len = strlen($str);
 
     for ($i = 0; $i < $len; $i++) {
-        $in = ord($str{$i});
+        $in = ord($str[$i]);
 
         if ($mState == 0) {
             // When mState is zero we expect either a US-ASCII character or a
diff --git a/phputf8/utils/validation.php b/phputf8/utils/validation.php
index dac1d0ae..13c89ada 100644
--- a/phputf8/utils/validation.php
+++ b/phputf8/utils/validation.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * Part of Windwalker project.
  *
@@ -32,7 +32,7 @@ function utf8_is_valid($str)
     $len = strlen($str);
 
     for ($i = 0; $i < $len; $i++) {
-        $in = ord($str{$i});
+        $in = ord($str[$i]);
 
         if ($mState == 0) {
             // When mState is zero we expect either a US-ASCII character or a