There are a few options:
if (substr($string, -1) == ',') {
Or (slightly less readable):
if ($string[strlen($string) - 1] == ',') {
Or (even less readable):
if (strrpos($string, ',') == strlen($string) - 1) {
Or (even worse yet):
if (preg_match('/,$/', $string)) {
Or (wow this is bad):
if (end(explode(',', $string)) == '') {
The take away, is just use substr($string, -1)
and be done with it. But there are many other alternatives out there...