Like $php_errormsg, the return value of this function may not be updated if a user-defined error handler returns non-FALSE. Tested on PHP 5.2.6.
<?php
var_dump(PHP_VERSION);
// Outputs: string(5) "5.2.6"
@trigger_error("foo");
$e=error_get_last();
var_dump($e['message']);
// Outputs: string(3) "foo"
set_error_handler(create_function('$a,$b',''));
@trigger_error("bar");
$e=error_get_last();
var_dump($e['message']);
// Outputs: string(3) "foo"
set_error_handler(create_function('$a,$b','return false;'));
@trigger_error("baz");
$e=error_get_last();
var_dump($e['message']);
// Outputs: string(3) "baz"
?>