bug #18352 [Debug] Fix case sensitivity checks (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
----------

[Debug] Fix case sensitivity checks

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #18344
| License       | MIT
| Doc PR        | -

Commits
-------

7336177 [Debug] Fix case sensitivity checks

src/Symfony/Component/Debug/DebugClassLoader.php

@@ -51,15 +51,25 @@ public function __construct($classLoader)
         }
 
         if (!isset(self::$caseCheck)) {
-            if(!file_exists(strtolower(__FILE__))) {
+            $file = file_exists(__FILE__) ? __FILE__ : rtrim(realpath('.'), DIRECTORY_SEPARATOR);
+            $i = strrpos($file, DIRECTORY_SEPARATOR);
+            $dir = substr($file, 0, 1 + $i);
+            $file = substr($file, 1 + $i);
+            $test = strtoupper($file) === $file ? strtolower($file) : strtoupper($file);
+            $test = realpath($dir.$test);
+
+            if (false === $test || false === $i) {
                 // filesystem is case sensitive
                 self::$caseCheck = 0;
-            } elseif(realpath(strtolower(__FILE__)) === __FILE__) {
-                // filesystem is not case sensitive
+            } elseif (substr($test, -strlen($file)) === $file) {
+                // filesystem is case insensitive and realpath() normalizes the case of characters
                 self::$caseCheck = 1;
-            } else {
-                // filesystem is not case sensitive AND realpath() fails to normalize case
+            } elseif (false !== stripos(PHP_OS, 'darwin')) {
+                // on MacOSX, HFS+ is case insensitive but realpath() doesn't normalize the case of characters
                 self::$caseCheck = 2;
+            } else {
+                // filesystem case checks failed, fallback to disabling them
+                self::$caseCheck = 0;
             }
         }
     }