PHP create random string

function random_string($type = 'alnum', $len = 8)
{					
	switch($type)
	{
		case 'alnum'	:
		case 'numeric'	:
		case 'nozero'	:
		
				switch ($type)
				{
					case 'alnum'	:	$pool = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
						break;
					case 'numeric'	:	$pool = '0123456789';
						break;
					case 'nozero'	:	$pool = '123456789';
						break;
				}

				$str = '';
				for ($i=0; $i < $len; $i++)
				{
					$str .= substr($pool, mt_rand(0, strlen($pool) -1), 1);
				}
				return $str;
		  break;
		case 'unique' : return md5(uniqid(mt_rand()));
		  break;
	}
}