diff --git a/.github/workflows/continuous-integration.yml b/.github/workflows/continuous-integration.yml
index 94079d6..91afbff 100644
--- a/.github/workflows/continuous-integration.yml
+++ b/.github/workflows/continuous-integration.yml
@@ -19,9 +19,6 @@ jobs:
     strategy:
       matrix:
         php:
-          - "5.6"
-          - "7.0"
-          - "7.1"
           - "7.2"
           - "7.3"
           - "7.4"
diff --git a/.php-cs-fixer.php b/.php-cs-fixer.php
new file mode 100644
index 0000000..460eecd
--- /dev/null
+++ b/.php-cs-fixer.php
@@ -0,0 +1,30 @@
+<?php
+
+$finder = (new PhpCsFixer\Finder())
+    ->in(__DIR__)
+    ->exclude(['vendor'])
+;
+
+return (new PhpCsFixer\Config())
+    ->setRiskyAllowed(true)
+    ->setRules([
+        '@Symfony' => true,
+        '@Symfony:risky' => true,
+        'array_syntax' => ['syntax' => 'short'],
+        'combine_consecutive_unsets' => true,
+        'heredoc_to_nowdoc' => true,
+        'no_extra_blank_lines' => ['tokens' => ['break', 'continue', 'extra', 'return', 'throw', 'use', 'parenthesis_brace_block', 'square_brace_block', 'curly_brace_block']],
+        'no_unreachable_default_argument_value' => true,
+        'no_useless_else' => true,
+        'no_useless_return' => true,
+        'ordered_class_elements' => true,
+        'ordered_imports' => true,
+        'php_unit_strict' => true,
+        'phpdoc_order' => true,
+        // 'psr4' => true,
+        'strict_comparison' => true,
+        'strict_param' => true,
+        'concat_space' => ['spacing' => 'one'],
+    ])
+    ->setFinder($finder)
+;
diff --git a/.php_cs b/.php_cs
deleted file mode 100644
index 5184eaf..0000000
--- a/.php_cs
+++ /dev/null
@@ -1,45 +0,0 @@
-<?php
-
-return PhpCsFixer\Config::create()
-    ->setRiskyAllowed(true)
-    ->setRules([
-        '@Symfony' => true,
-        '@Symfony:risky' => true,
-        'array_syntax' => [
-            'syntax' => 'short'
-        ],
-        'combine_consecutive_unsets' => true,
-        'heredoc_to_nowdoc' => true,
-        'no_extra_consecutive_blank_lines' => [
-        'break',
-            'continue',
-            'extra',
-            'return',
-            'throw',
-            'use',
-            'parenthesis_brace_block',
-            'square_brace_block',
-            'curly_brace_block'
-        ],
-        'no_unreachable_default_argument_value' => true,
-        'no_useless_else' => true,
-        'no_useless_return' => true,
-        'ordered_class_elements' => true,
-        'ordered_imports' => true,
-        'php_unit_strict' => true,
-        'phpdoc_order' => true,
-        // 'psr4' => true,
-        'strict_comparison' => true,
-        'strict_param' => true,
-        'concat_space' => [
-            'spacing' => 'one'
-        ],
-    ])
-    ->setFinder(
-        PhpCsFixer\Finder::create()
-            ->exclude([
-                'vendor',
-            ])
-            ->in(__DIR__)
-    )
-;
diff --git a/README.md b/README.md
index b492a05..01af9d3 100644
--- a/README.md
+++ b/README.md
@@ -14,9 +14,8 @@ Uses [Imgur API v3](https://api.imgur.com/).
 
 * Branch [1.x](https://github.com/j0k3r/php-imgur-api-client/tree/1.x) use Guzzle 3 (but is not maintained)
 * Branch [2.x](https://github.com/j0k3r/php-imgur-api-client/tree/2.x) use Guzzle 5 (but is not maintained)
-* Branch [master](https://github.com/j0k3r/php-imgur-api-client/tree/master) use Guzzle 6
-
-All actives branches required PHP >= 5.6
+* Branch [3.x](https://github.com/j0k3r/php-imgur-api-client/tree/3.x) use Guzzle 6 and PHP >= 5.6
+* Branch [master](https://github.com/j0k3r/php-imgur-api-client/tree/master) use Guzzle 7 and PHP >= 7.2.5
 
 ## Composer
 
@@ -28,10 +27,8 @@ $ curl -s http://getcomposer.org/installer | php
 
 Add the library details to your composer.json
 
-```json
-"require": {
-    "j0k3r/php-imgur-api-client": "^3.0.0"
-}
+```bash
+composer require j0k3r/php-imgur-api-client@^4.0
 ```
 
 Install the dependency with
diff --git a/composer.json b/composer.json
index d433d73..135988e 100644
--- a/composer.json
+++ b/composer.json
@@ -34,13 +34,13 @@
         }
     },
     "require": {
-        "php": ">=5.6.0",
+        "php": ">=7.2.5",
         "ext-curl": "*",
-        "guzzlehttp/guzzle": "^6.0"
+        "guzzlehttp/guzzle": "^7.0"
     },
     "require-dev": {
-        "friendsofphp/php-cs-fixer": "^2.0",
-        "symfony/phpunit-bridge": "^5.0"
+        "friendsofphp/php-cs-fixer": "^3.0",
+        "symfony/phpunit-bridge": "^6.0"
     },
     "config": {
         "sort-packages": true
diff --git a/lib/Imgur/Auth/OAuth2.php b/lib/Imgur/Auth/OAuth2.php
index 5da0ce3..38752e5 100644
--- a/lib/Imgur/Auth/OAuth2.php
+++ b/lib/Imgur/Auth/OAuth2.php
@@ -12,8 +12,8 @@
  */
 class OAuth2 implements AuthInterface
 {
-    const AUTHORIZATION_ENDPOINT = 'https://api.imgur.com/oauth2/authorize';
-    const ACCESS_TOKEN_ENDPOINT = 'https://api.imgur.com/oauth2/token';
+    public const AUTHORIZATION_ENDPOINT = 'https://api.imgur.com/oauth2/authorize';
+    public const ACCESS_TOKEN_ENDPOINT = 'https://api.imgur.com/oauth2/token';
 
     /**
      * Indicates the client that is making the request.