<?php
$str = 'Old string Old more string Old some more string';
$old = 'Old';
$new = 'New';
$olds = Array();
$news = Array();
$n = substr_count($str, $old);
for ($i = 0; $i < $n; $i++) {
$olds[] = $old;
$news[] = $new . ($i+1);
}
echo str_replace($olds, $news, $str);
?>