Description:
------------
A custom error handler set with set_error_handler() cannot be removed.

The documentation says that the return value of set_error_handler() is the previous error handler. In case of no previous error handler, NULL is returned.

However set_error_handler() cannot be called with NULL to restore the PHP default error handler. This results in the following:

$ php -r 'set_error_handler(NULL);'
Warning: set_error_handler() expects argument 1, '', to be a valid callback in Command line code on line 1


Reproduce code:
---------------
<?php
    function handler() {
        echo "called\n";
    }

    set_error_handler('handler called');
    set_error_handler();
?>


Expected result:
----------------
Somehow a way for removing the current error handler and restoring the original behaviour should be possible.

Actual result:
--------------
handler called