diff --git a/src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_4_layout.html.twig b/src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_4_layout.html.twig
index b4e5b0120eebf..8c6414895b88c 100644
--- a/src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_4_layout.html.twig
+++ b/src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_4_layout.html.twig
@@ -137,6 +137,11 @@
     {{- parent() -}}
 {%- endblock button_widget %}
 
+{% block submit_widget -%}
+    {%- set attr = attr|merge({class: (attr.class|default('btn-primary'))|trim}) -%}
+    {{- parent() -}}
+{%- endblock submit_widget %}
+
 {% block checkbox_widget -%}
     {%- set parent_label_class = parent_label_class|default(label_attr.class|default('')) -%}
     {%- if 'checkbox-custom' in parent_label_class -%}
diff --git a/src/Symfony/Component/Dotenv/Dotenv.php b/src/Symfony/Component/Dotenv/Dotenv.php
index 41b83f57a64d1..025206326a8bb 100644
--- a/src/Symfony/Component/Dotenv/Dotenv.php
+++ b/src/Symfony/Component/Dotenv/Dotenv.php
@@ -66,8 +66,8 @@ public function load($path/*, ...$paths*/)
      */
     public function populate($values)
     {
-        $loadedVars = array_flip(explode(',', getenv('SYMFONY_DOTENV_VARS')));
-        unset($loadedVars['']);
+        $updateLoadedVars = false;
+        $loadedVars = array_flip(explode(',', isset($_SERVER['SYMFONY_DOTENV_VARS']) ? $_SERVER['SYMFONY_DOTENV_VARS'] : (isset($_ENV['SYMFONY_DOTENV_VARS']) ? $_ENV['SYMFONY_DOTENV_VARS'] : '')));
 
         foreach ($values as $name => $value) {
             $notHttpName = 0 !== strpos($name, 'HTTP_');
@@ -82,14 +82,15 @@ public function populate($values)
                 $_SERVER[$name] = $value;
             }
 
-            $loadedVars[$name] = true;
+            if (!isset($loadedVars[$name])) {
+                $loadedVars[$name] = $updateLoadedVars = true;
+            }
         }
 
-        if ($loadedVars) {
+        if ($updateLoadedVars) {
+            unset($loadedVars['']);
             $loadedVars = implode(',', array_keys($loadedVars));
-            putenv("SYMFONY_DOTENV_VARS=$loadedVars");
-            $_ENV['SYMFONY_DOTENV_VARS'] = $loadedVars;
-            $_SERVER['SYMFONY_DOTENV_VARS'] = $loadedVars;
+            putenv('SYMFONY_DOTENV_VARS='.$_ENV['SYMFONY_DOTENV_VARS'] = $_SERVER['SYMFONY_DOTENV_VARS'] = $loadedVars);
         }
     }
 
diff --git a/src/Symfony/Component/Dotenv/Tests/DotenvTest.php b/src/Symfony/Component/Dotenv/Tests/DotenvTest.php
index 7caa75f06622b..4492c8e9e08e0 100644
--- a/src/Symfony/Component/Dotenv/Tests/DotenvTest.php
+++ b/src/Symfony/Component/Dotenv/Tests/DotenvTest.php
@@ -268,7 +268,7 @@ public function testMemorizingLoadedVarsNamesInSpecialVar()
 
     public function testOverridingEnvVarsWithNamesMemorizedInSpecialVar()
     {
-        putenv('SYMFONY_DOTENV_VARS=FOO,BAR,BAZ');
+        putenv('SYMFONY_DOTENV_VARS='.$_SERVER['SYMFONY_DOTENV_VARS'] = 'FOO,BAR,BAZ');
 
         putenv('FOO=foo');
         putenv('BAR=bar');
diff --git a/src/Symfony/Component/HttpFoundation/ParameterBag.php b/src/Symfony/Component/HttpFoundation/ParameterBag.php
index e3be0a9b8b22a..19d7ee913a253 100644
--- a/src/Symfony/Component/HttpFoundation/ParameterBag.php
+++ b/src/Symfony/Component/HttpFoundation/ParameterBag.php
@@ -174,7 +174,7 @@ public function getInt($key, $default = 0)
      * Returns the parameter value converted to boolean.
      *
      * @param string $key     The parameter key
-     * @param mixed  $default The default value if the parameter key does not exist
+     * @param bool   $default The default value if the parameter key does not exist
      *
      * @return bool The filtered value
      */
diff --git a/src/Symfony/Component/HttpFoundation/Response.php b/src/Symfony/Component/HttpFoundation/Response.php
index 832260a51e5e8..f267d1e0e0fcc 100644
--- a/src/Symfony/Component/HttpFoundation/Response.php
+++ b/src/Symfony/Component/HttpFoundation/Response.php
@@ -334,8 +334,9 @@ public function sendHeaders()
 
         // headers
         foreach ($this->headers->allPreserveCaseWithoutCookies() as $name => $values) {
+            $replace = 0 === strcasecmp($name, 'Content-Type');
             foreach ($values as $value) {
-                header($name.': '.$value, false, $this->statusCode);
+                header($name.': '.$value, $replace, $this->statusCode);
             }
         }