来自维基百科：

$ends = array('th','st','nd','rd','th','th','th','th','th','th');
if (($number %100) >= 11 && ($number%100) <= 13)
   $abbreviation = $number. 'th';
else
   $abbreviation = $number. $ends[$number % 10];

$number你要写的数字在哪里。适用于任何自然数。

作为一个函数：

function ordinal($number) {
    $ends = array('th','st','nd','rd','th','th','th','th','th','th');
    if ((($number % 100) >= 11) && (($number%100) <= 13))
        return $number. 'th';
    else
        return $number. $ends[$number % 10];
}
//Example Usage
echo ordinal(100);