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/specials/SpecialRunJobs.php:                   set_error_handler( function ( $errno, $errstr ) {
```
includes/libs/ScopedCallback.php

```
$ git submodule foreach 'git grep error_handler || true'                                                                                                            
Entering 'extensions/CodeEditor'
modules/ace/mode-php.js:        'resourcebundle|restore_error_handler|restore_exception_handler|restore_include_path|return|rewind|rewinddir|rmdir|round|rpm_close|' +
modules/ace/mode-php.js:        'session_unregister|session_unset|session_write_close|setCounterClass|set_error_handler|set_exception_handler|set_file_buffer|' +
modules/ace/mode-php.js:        'urldecode|urlencode|use_soap_error_handler|user_error|usleep|usort|utf8_decode|utf8_encode|v8js|v8jsexception|var_dump|var_export|variant|' +
modules/ace/mode-php.js:        'xslt_error|xslt_free|xslt_getopt|xslt_process|xslt_set_base|xslt_set_encoding|xslt_set_error_handler|xslt_set_log|xslt_set_object|' +
modules/ace/mode-php.js:    "restore_error_handler": [
modules/ace/mode-php.js:        "void restore_error_handler(void)",
modules/ace/mode-php.js:    "set_error_handler": [
modules/ace/mode-php.js:        "string set_error_handler(string error_handler [, int error_types])",
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:        $this->assertEquals(array(Handler::getInstance(), 'handleError'), set_error_handler(function () {
pear/pear-core-minimal/src/PEAR.php:$GLOBALS['_PEAR_error_handler_stack']    = array();
pear/pear-core-minimal/src/PEAR.php:    var $_default_error_handler = '';
pear/pear-core-minimal/src/PEAR.php:        $stack       = &$GLOBALS['_PEAR_error_handler_stack'];
pear/pear-core-minimal/src/PEAR.php:        $stack = &$GLOBALS['_PEAR_error_handler_stack'];
pear/pear-core-minimal/src/PEAR.php:        $stack = &$GLOBALS['_PEAR_error_handler_stack'];
pear/pear-core-minimal/src/PEAR.php:        $stack = &$GLOBALS['_PEAR_error_handler_stack'];
pear/pear-core-minimal/src/System.php:* {@link set_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();
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();
```