As of c8c789ba64d80f1cfbeb6ee479dfd6e73d71aa62.
```
jan@dforrest:~/projects/wikimedia/mediawiki/deploy (wmf/1.27.0-wmf.21 %>)$ git grep error_handler
includes/filebackend/FSFileBackend.php:         set_error_handler( [ $this, 'handleWarning' ], E_WARNING );
includes/filebackend/FSFileBackend.php:         restore_error_handler(); // restore previous handler
```
includes/libs/ScopedCallback.php

```
$ git submodule foreach 'git grep error_handler || true'                                                                                                            
Entering 'extensions/Echo'
includes/controller/NotificationController.php:         set_error_handler( array( __CLASS__, 'formatterErrorHandler' ), -1 );
includes/controller/NotificationController.php:         restore_error_handler();
Entering 'extensions/Flow'
Hooks.php:              set_error_handler( new Flow\RecoverableErrorHandler, -1 );
Hooks.php:              restore_error_handler();
Hooks.php:              set_error_handler( new Flow\RecoverableErrorHandler, -1 );
Hooks.php:                              restore_error_handler();
Hooks.php:                      restore_error_handler();
Hooks.php:              restore_error_handler();
Hooks.php:              set_error_handler( new Flow\RecoverableErrorHandler, -1 );
Hooks.php:                              restore_error_handler();
Hooks.php:                      restore_error_handler();
Hooks.php:              restore_error_handler();
Hooks.php:              set_error_handler( new Flow\RecoverableErrorHandler, -1 );
Hooks.php:              restore_error_handler();
Hooks.php:              set_error_handler( new Flow\RecoverableErrorHandler, -1 );
Hooks.php:              restore_error_handler();
Hooks.php:              set_error_handler( new Flow\RecoverableErrorHandler, -1 );
Hooks.php:              restore_error_handler();
Hooks.php:              set_error_handler( new Flow\RecoverableErrorHandler, -1 );
Hooks.php:              restore_error_handler();
Hooks.php:              set_error_handler( new Flow\RecoverableErrorHandler, -1 );
Hooks.php:              restore_error_handler();
Hooks.php:              set_error_handler( new Flow\RecoverableErrorHandler, -1 );
Hooks.php:              restore_error_handler();
includes/RecoverableErrorHandler.php: *  set_error_handler( new RecoverableErrorHandler, E_RECOVERABLE_ERROR );
includes/RecoverableErrorHandler.php: *  restore_error_handler();
Entering 'vendor'
monolog/monolog/src/Monolog/ErrorHandler.php:        $prev = set_
(array($this, 'handleError'), $errorTypes);
monolog/monolog/src/Monolog/Handler/RotatingFileHandler.php:                set_error_handler(function ($errno, $errstr, $errfile, $errline) {});
monolog/monolog/src/Monolog/Handler/RotatingFileHandler.php:                restore_error_handler();
monolog/monolog/src/Monolog/Handler/StreamHandler.php:            set_error_handler(array($this, 'customErrorHandler'));
monolog/monolog/src/Monolog/Handler/StreamHandler.php:            restore_error_handler();
monolog/monolog/src/Monolog/Handler/StreamHandler.php:            set_error_handler(array($this, 'customErrorHandler'));
monolog/monolog/src/Monolog/Handler/StreamHandler.php:            restore_error_handler();
monolog/monolog/tests/Monolog/Formatter/NormalizerFormatterTest.php:        set_error_handler(function ($level, $message, $file, $line, $context) use ($that) {
monolog/monolog/tests/Monolog/Formatter/NormalizerFormatterTest.php:                restore_error_handler();
monolog/monolog/tests/Monolog/Formatter/NormalizerFormatterTest.php:        restore_error_handler();
monolog/monolog/tests/Monolog/Formatter/NormalizerFormatterTest.php:        set_error_handler(function ($level, $message, $file, $line, $context) use ($that) {
monolog/monolog/tests/Monolog/Formatter/NormalizerFormatterTest.php:                restore_error_handler();
monolog/monolog/tests/Monolog/Formatter/NormalizerFormatterTest.php:        restore_error_handler();
monolog/monolog/tests/Monolog/Formatter/NormalizerFormatterTest.php:        set_error_handler(function ($errno, $errstr, $errfile, $errline) {
monolog/monolog/tests/Monolog/Formatter/NormalizerFormatterTest.php:            restore_error_handler();
monolog/monolog/tests/Monolog/Handler/PHPConsoleHandlerTest.php:        $this->assertEquals(array(Handler::getInstance(), 'handleError'), set_error_handler(function () {
wikimedia/php-session-serializer/src/Wikimedia/PhpSessionSerializer.php:                set_error_handler( function ( $errno, $errstr ) use ( &$error ) {
wikimedia/php-session-serializer/src/Wikimedia/PhpSessionSerializer.php:                restore_error_handler();
wikimedia/php-session-serializer/src/Wikimedia/PhpSessionSerializer.php:                set_error_handler( function ( $errno, $errstr ) use ( &$error ) {
wikimedia/php-session-serializer/src/Wikimedia/PhpSessionSerializer.php:                restore_error_handler();
```