php怎样求一个数组中最长的

<?php

$arr = array
(
    0 => 'd',
    1 => '68b3',
    2 => 'a86',
    3 => 'c9aa97b23b71d5c',
    4 => '55169',
);
echo get_max_val($arr);

function get_max_val($arr)
{
	$key = null;
	foreach ($arr as $k=>$v)
	{
		if($key == null || strval($v) > strval($arr[$key]))
		{
			$key = $k;
		}
	}
	return $key;
}

?>