<?php
  function str_insert($str, $search, $insert) {
    $index = strpos($str, $search);
    if($index === false) {
        return $str;
    }
    return substr_replace($str, $search.$insert, $index, strlen($search));
  }

  echo str_insert("The big brown dog jumped over the fence", "dog", "s");

The big brown dogs jumped over the fence