Added an PHP CS fixer config file and applied it to files.

We now use the same the same style as the symfony project, and it allows us to simply fix the style by executing php_cs_fixer fix in the project root.

.php_cs.dist

@@ -0,0 +1,30 @@
+<?php
+
+/**
+ * Based on the .php_cs.dist of the symfony project
+ * https://github.com/symfony/symfony/blob/4.4/.php_cs.dist
+ */
+
+if (!file_exists(__DIR__.'/src')) {
+    exit(0);
+}
+
+return PhpCsFixer\Config::create()
+    ->setRules([
+        '@Symfony' => true,
+        '@Symfony:risky' => true,
+        '@PHPUnit75Migration:risky' => true,
+        'php_unit_dedicate_assert' => ['target' => '5.6'],
+        'array_syntax' => ['syntax' => 'short'],
+        'fopen_flags' => false,
+        'protected_to_private' => false,
+        'combine_nested_dirname' => true,
+    ])
+    ->setRiskyAllowed(true)
+    ->setFinder(
+        PhpCsFixer\Finder::create()
+            ->in(__DIR__.'/src')
+            ->in(__DIR__.'/tests')
+            ->append([__FILE__])
+    )
+    ;