diff --git a/README.md b/README.md
index 7c75549..22561b4 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@ ConvertAPI helps in converting various file formats. Creating PDF and Images fro
 
 ## Requirements
 
-PHP 5.4.0 and later.
+PHP 8.1.0 and later.
 
 ## Installation
 
diff --git a/examples/conversions_chaining.php b/examples/conversions_chaining.php
index 2e724d6..de4593f 100644
--- a/examples/conversions_chaining.php
+++ b/examples/conversions_chaining.php
@@ -18,14 +18,14 @@
 $cost = $jpgResult->getConversionCost();
 $count = count($jpgResult->getFiles());
 
-echo "Conversions done. Cost: ${cost}. Total files created: ${count}\n";
+echo "Conversions done. Cost: {$cost}. Total files created: {$count}\n";
 
 $zipResult = ConvertApi::convert('zip', ['Files' => $jpgResult->getFiles()], 'any');
 
 $cost = $zipResult->getConversionCost();
 $count = count($zipResult->getFiles());
 
-echo "Conversions done. Cost: ${cost}. Total files created: ${count}\n";
+echo "Conversions done. Cost: {$cost}. Total files created: {$count}\n";
 
 $savedFiles = $zipResult->saveFiles($dir);
 
diff --git a/lib/ConvertApi/Client.php b/lib/ConvertApi/Client.php
index ed4513b..2bdef4a 100644
--- a/lib/ConvertApi/Client.php
+++ b/lib/ConvertApi/Client.php
@@ -44,7 +44,7 @@ public function upload($file_or_resource, $fileName)
 
         if (!$fp)
         {
-            throw new Error\File("Unable to open file ${file_or_resource}");
+            throw new Error\File("Unable to open file {$file_or_resource}");
         }
 
         curl_setopt($ch, CURLOPT_POST, true);
@@ -72,7 +72,7 @@ public function download($url, $path)
 
         if (!$fp)
         {
-            throw new Error\File("Unable to open file ${path}");
+            throw new Error\File("Unable to open file {$path}");
         }
 
         curl_setopt($ch, CURLOPT_FILE, $fp);
@@ -193,7 +193,7 @@ private function buildFormData($params)
             if (is_array($val))
             {
                 foreach ($val as $k => $v)
-                    $data["${key}[${k}]"] = $v;
+                    $data["{$key}[{$k}]"] = $v;
             }
             elseif (is_bool($val))
                 $data[$key] = $val ? 'true' : 'false';
diff --git a/lib/ConvertApi/FileUpload.php b/lib/ConvertApi/FileUpload.php
index 5779157..11779c3 100644
--- a/lib/ConvertApi/FileUpload.php
+++ b/lib/ConvertApi/FileUpload.php
@@ -4,6 +4,10 @@
 
 class FileUpload
 {
+    public $filePath;
+    private string $_fileName;
+    private array $_result;
+
     function __construct($filePath, $fileName = null)
     {
         $this->filePath = $filePath;
diff --git a/lib/ConvertApi/FormatDetector.php b/lib/ConvertApi/FormatDetector.php
index 41bc458..e93d30d 100644
--- a/lib/ConvertApi/FormatDetector.php
+++ b/lib/ConvertApi/FormatDetector.php
@@ -4,6 +4,8 @@
 
 class FormatDetector
 {
+    private $resource;
+
     function __construct($resource)
     {
         $this->resource = $resource;
diff --git a/lib/ConvertApi/Result.php b/lib/ConvertApi/Result.php
index f719131..571977e 100644
--- a/lib/ConvertApi/Result.php
+++ b/lib/ConvertApi/Result.php
@@ -4,7 +4,8 @@
 
 class Result
 {
-    private $files;
+    private array $files;
+    public array $response;
 
     function __construct($response)
     {
diff --git a/lib/ConvertApi/ResultFile.php b/lib/ConvertApi/ResultFile.php
index e3c9fe9..e84ec62 100644
--- a/lib/ConvertApi/ResultFile.php
+++ b/lib/ConvertApi/ResultFile.php
@@ -4,6 +4,8 @@
 
 class ResultFile
 {
+    public array $fileInfo;
+
     function __construct($fileInfo)
     {
         $this->fileInfo = $fileInfo;
diff --git a/lib/ConvertApi/Task.php b/lib/ConvertApi/Task.php
index fca394e..29baf1d 100644
--- a/lib/ConvertApi/Task.php
+++ b/lib/ConvertApi/Task.php
@@ -6,6 +6,11 @@ class Task
 {
     const DEFAULT_URL_FORMAT = 'url';
 
+    private ?string $fromFormat;
+    private ?string $toFormat;
+    private array $params;
+    private ?int $conversionTimeout;
+
     function __construct($fromFormat, $toFormat, $params, $conversionTimeout = null)
     {
         $this->fromFormat = $fromFormat;