If you want a string instead of an array of numbers, you can use the join method http://php.net/manual/en/function.join.php.
$str = join(',', array(1, 2, 3));
If you are wanting to create an array out of a string like ‘1,2,3’ you could use the explode method http://php.net/manual/en/function.explode.php.
$arr = explode(",", "1,2,3");
solved How to put PHP variable in array