php function count thai string

function strlenth($str)
{
$arr = str_split($str);
$count = 0;
	foreach($arr as $val)
	{
		$ascii = ord($val);
		if(!( $ascii == 209 ||  ($ascii >= 212 && $ascii <= 218 ) || ($ascii >= 231 && $ascii <= 238 )))
			$count += 1;
	}
	return $count;
}

$str = "นครราชสีมา ครับ";
echo strlenth($str);