minor symfony#20334 [PhpUnitBridge] Fix undefined variable (romainneutron)

This PR was merged into the 3.1 branch.

Discussion
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[PhpUnitBridge] Fix undefined variable

| Q             | A
| ------------- | ---
| Branch?       | 3.1
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT

Otherwise `$mode` is not defined on line 115

Commits
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c4cc392 [PhpUnitBridge] Fix undefined variable

Author: nicolas-grekas

src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler.php

@@ -68,7 +68,8 @@ public static function register($mode = 0)
             'other' => array(),
         );
         $deprecationHandler = function ($type, $msg, $file, $line, $context) use (&$deprecations, $getMode) {
-            if (E_USER_DEPRECATED !== $type || DeprecationErrorHandler::MODE_DISABLED === $mode = $getMode()) {
+            $mode = $getMode();
+            if (E_USER_DEPRECATED !== $type || DeprecationErrorHandler::MODE_DISABLED === $mode) {
                 return \PHPUnit_Util_ErrorHandler::handleError($type, $msg, $file, $line, $context);
             }