<?php
$file = '[STRING_****] (STRING_*****) /STRING';
$searchContext = 'STRING';
$replaceContext = 'NEW_STRING';
$result = preg_replace('/[\[(\s]\K'.$searchContext.'/', $replaceContext, $file); echo $result;
PD9waHAKCiRmaWxlICAgID0gJ1tTVFJJTkdfKioqKl0gIChTVFJJTkdfKioqKiopICAvU1RSSU5HJzsKJHNlYXJjaENvbnRleHQgID0gJ1NUUklORyc7CiRyZXBsYWNlQ29udGV4dCA9ICdORVdfU1RSSU5HJzsKJHJlc3VsdCA9IHByZWdfcmVwbGFjZSgnL1tcWyhcc11cSycuJHNlYXJjaENvbnRleHQuJy8nLCAkcmVwbGFjZUNvbnRleHQsICRmaWxlKTsKZWNobyAkcmVzdWx0Ow==