Fatal during setup for "mbstring.func_overload"

Summary:
Fixes T5545. We assume `strlen()` returns the number of bytes in a string, which is the normal behavior (and the documented behavior).

There's a config option, `mbstring.func_overload`, which silently calls mb_strlen() instead. This may return some other result, might fail, etc., and there's no way to get the byte length of a string if this option is set.

If this option is set, fatal immediately. Nothing good can ever come of it.

Test Plan: {F173990}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5545

Differential Revision: https://secure.phabricator.com/D9811

src/applications/config/check/PhabricatorSetupCheckPHPConfig.php

@@ -75,6 +75,23 @@ final class PhabricatorSetupCheckPHPConfig extends PhabricatorSetupCheck {
       }
     }
 
+    $overload_option = 'mbstring.func_overload';
+    $func_overload = ini_get($overload_option);
+    if ($func_overload) {
+      $message = pht(
+        "You have '%s' enabled in your PHP configuration.\n\n".
+        "This option is not compatible with Phabricator. Disable ".
+        "'%s' in your PHP configuration to continue.",
+        $overload_option,
+        $overload_option);
+
+      $this->newIssue('php'.$overload_option)
+        ->setIsFatal(true)
+        ->setName(pht('Disable PHP %s', $overload_option))
+        ->setMessage($message)
+        ->addPHPConfig($overload_option);
+    }
+
     $open_basedir = ini_get('open_basedir');
     if ($open_basedir) {