If you would like a different solution using functional programming coding style, here it is:
<?php
$str = 'a,b,c,d,e';
$add_quotes = function($str, $func) {
return implode(',', array_map($func, explode(',', $str)));
};
print $add_quotes(
$str,
function ($a) {
return "'$a'";
}
);