<?php
function string($string) {
  $arraySize = count($string);

  for($x = 0; $x < $arraySize; $x++) {
    if ($x < $arraySize-1) {
      echo $string.",";
    } else {
      echo $string;
    }
  }
  return $string;
}

$string = "this,is,a,test";
echo string($string);
?>