Contrary to the question asked, rtrim() will remove any number of characters, listed in the second argument, from the end of the string. In case you expect just a single comma, the following code would do:
$newarraynama = rtrim($arraynama, ",");
But in my case I had 2 characters, a comma and a space, so I had to change to
$newarraynama = rtrim($arraynama, " ,");
and now it would remove all commas and spaces from the end of the string, returning a, b, c, d, e either from a, b, c, d, e,, a, b, c, d, e,,,, a, b, c, d, e, or a, b, c, d, e , ,, , ,
But in case there could be multiple commas but you need to remove only the last one, then rtrim() shouldn’t be used at all – see other answers for the solution that directly answers the question.
However, rtrim() could be a good choice if you don’t know whether the extra character could be present or not. Unlike substr-based solutions it will return a, b, c, d, e from a, b, c, d, e