<?php
$file = '[STRING_****] (STRING_*****) /STRING';
$searchContext = 'STRING';
$replaceContext = 'NEW_STRING';
$pattern = '/(?<=[\[(\s])'.$searchContext.'/';
echo $result;
PD9waHAKCiRmaWxlICAgID0gJ1tTVFJJTkdfKioqKl0gIChTVFJJTkdfKioqKiopICAvU1RSSU5HJzsKJHNlYXJjaENvbnRleHQgID0gJ1NUUklORyc7CiRyZXBsYWNlQ29udGV4dCA9ICdORVdfU1RSSU5HJzsKJHBhdHRlcm4gPSAnLyg/PD1bXFsoXHNdKScuJHNlYXJjaENvbnRleHQuJy8nOwokcmVzdWx0ID0gcHJlZ19yZXBsYWNlKCRwYXR0ZXJuLCAkcmVwbGFjZUNvbnRleHQsICRmaWxlKTsKZWNobyAkcmVzdWx0Ow==