<?php
$string = 'III.OOO.OOO.OOO.S';
$search = 'XXX';
$replace = 'B';
//replace ONLY FIRST occurance of "OOO" with "B"
echo substr_replace($string,$replace,0,strlen($search));
//$string => B.OOO.OOO.S
echo "\n---\n";
//replace ONLY LAST occurance of "OOOO" with "B"
echo substr_replace($string,$replace,strrpos($string,$search),strlen($search));
//$string => OOO.OOO.B.S
echo "\n---\n";
//replace ONLY LAST occurance of "OOOO" with "B"
echo strrev(implode(strrev($replace),explode(strrev($search),strrev($string),2)));
//$string => OOO.OOO.B.S