I've been having the same issue with this similar problem recently. I fixed it by using an increment variable $i, initializing it to 0, then having it increment inside the foreach loop. Within that loop place an if, else, with the echo statement including a comma if the $i counter is less than the sizeof() operator of your array/variable.
I don't know if this would fix your issue per se, but it helped me with mine. I realize this question is years-old, but hopefully this will help someone else. I'm fairly new to PHP so I didn't quite understand a lot of the Answers that were given before me, though they were quite insightful, particularly the implode one.
$i=0;
foreach ($results as $result) {
$i++;
if(sizeof($results) > $i) {
echo $result . ", ";
} else {
echo $result;
}
}