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

I think that you're missing to assign the variable back after the trim:

$s = '"india","usa","uk",';
$s = rtrim($s, ',');
// prints "india","usa","uk"
print $s;

Demo

Try before buy

rtrim($string, ","); removes a comma at the end of the string.

PHP.net rtrim

trim($string, ","); removes a comma at the beginning and end of a string.

PHP.net trim

You can also use implode if you're working with arrays:

PHP.net implode

Example from php.net:

<?php
    $array = array('lastname', 'email', 'phone');
    $comma_separated = implode(",", $array);

    echo $comma_separated; // lastname,email,phone

    // Empty string when using an empty array:
    var_dump(implode('hello', array())); // string(0) ""
?>