change_str.php

<?php

/**
 * Exemplo de uso:
 *
 * echo change_str(',', '|', 'a,b,c,d,e', 3);
 * >>> 'a,b,c,d|e'
 */

function change_str($str, $replacer, $string, $pos)
{
  $y=explode($str, $string, $pos+2);
  if (isset($y[$pos+1])){
    $y[$pos]=$y[$pos].$replacer.$y[$pos+1];
    array_pop($y);
  }
  $z=implode($str, $y);
  return $z;
}