What I mean is the following

<?php
    // first call returns nothing
    $ret1 = set_error_handler("unknown_function");
    var_dump($ret1);

    // second call returns "unknown_function"
    $ret2 = set_error_handler("another_unknown_function");
    var_dump($ret2);
?>

The documentation says set_error_handler() would return FALSE if an error occurs. Therefore, both of the above calls should return FALSE since setting the error handler to an unknown function is certainly an error.

As a consequence I am not able to check if the call to set_error_handler() has been successful.