<?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
Warning: PHP Startup: Unable to load dynamic library 'sodium.so' (tried: /usr/lib/php/8.3.5/modules/sodium.so (libsodium.so.23: cannot open shared object file: No such file or directory), /usr/lib/php/8.3.5/modules/sodium.so.so (/usr/lib/php/8.3.5/modules/sodium.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
B.OOO.OOO.OOO.S
---
B.OOO.OOO.OOO.S
---
III.OOO.OOO.OOO.S