Restore the previous error handler in PHP

/*w  w w .ja  v  a 2  s  .  co  m*/
<?php
    //custom error handler function
    function customError($errno, $errstr, $errfile, $errline){
       echo "<b>Custom error:</b> [$errno] $errstr<br />";
       echo " Error on line $errline in $errfile<br />";
    }

    //set user-defined error handler
    set_error_handler("customError");

    $test=2;

    //trigger error
    if ($test>1){
        trigger_error("A custom error has been triggered");
    }

    //restore built-in error handler
    restore_error_handler();

    //trigger error again
    if ($test>1){
        trigger_error("A custom error has been triggered");
    }
?>