<?php
$string = 'test 4 test 8 test 10 11';
$patterns = array();
$patterns[0] = '/4/';
$patterns[1] = '/8/';
$patterns[2] = '/10/';
$replacements = array();
$replacements[2] = '0';
$replacements[1] = '0';
$replacements[0] = '0';
echo preg_replace($patterns, $replacements, $string);
?>