src/ErrorHandler/ErrorHandler.php
public function unregister()
    {
        if (!$this->registered) {
            return;
        }
        /*
            set and restore error handler to determine the current error handler
        */
        $errHandlerCur = \set_error_handler(array($this, 'handleError'));
        \restore_error_handler();
        if ($errHandlerCur == array($this, 'handleError')) {
            // we are the current error handler
            \restore_error_handler();
        }
        /*
            set and restore exception handler to determine the current error handler
        */
        $exHandlerCur = \set_exception_handler(array($this, 'handleException'));
        \restore_exception_handler();
        if ($exHandlerCur == array($this, 'handleException')) {
            // we are the current exception handler
            \restore_exception_handler();
        }
        \ini_set('display_errors', $this->prevDisplayErrors);
        $this->prevErrorHandler = null;
        $this->prevExceptionHandler = null;
        $this->registered = false;  // used by $this->onShutdown()
        return;
    }
public function unregister()
    {
        if (!$this->registered) {
            return;
        }
        /*
            set and restore error handler to determine the current error handler
        */
        $errHandlerCur = \set_error_handler(array($this, 'handleError'));
        \restore_error_handler();
        if ($errHandlerCur == array($this, 'handleError')) {
            // we are the current error handler
            \restore_error_handler();
        }
        /*
            set and restore exception handler to determine the current error handler
        */
        $exHandlerCur = \set_exception_handler(array($this, 'handleException'));
        \restore_exception_handler();
        if ($exHandlerCur == array($this, 'handleException')) {
            // we are the current exception handler
            \restore_exception_handler();
        }
        \ini_set('display_errors', $this->prevDisplayErrors);
        $this->prevErrorHandler = null;
        $this->prevExceptionHandler = null;
        $this->registered = false;  // used by $this->onShutdown()
        return;
    }
un-register this error handler and shutdown function

Note:  PHP conspicuously lacks an unregister_shutdown_function function.
Technically this will still be registered, however:
$this->registered will be used to keep track of whether
we're "registered" or not and behave accordingly

@return void