Description:
------------
It seams like PHP doesn't keep the error_handler when a function is called which contains another trigger_error. This can lead to a quite unwanted behaviour on scripts that require an own error_handler.

Reproduce code:
---------------
<?php

function my_error_handler($error_number,$error_message,$error_filename,$error_line){
    echo 'Using my_error_handler: '.$error_message.'<br>';

    if ( $error_number == E_USER_NOTICE ){
        call_error();
    }
}
function call_error(){
    trigger_error('From call_error()', E_USER_WARNING);
}

set_error_handler('my_error_handler');

trigger_error('test', E_USER_NOTICE);
trigger_error('another test', E_USER_NOTICE);

?>

Expected result:
----------------
Using my_error_handler: test
Using my_error_handler: From call_error()
Using my_error_handler: another test
Using my_error_handler: From call_error()

Actual result:
--------------
Using my_error_handler: test
Warning: From call_error() in /www/test.php on line 11
Using my_error_handler: another test
Warning: From call_error() in /www/test.php on line 11