<?php
$my_string = "newtext!234@@random_text@@weludud";
$new_text = 'replaced_text';
$var = explode('@@',$my_string); //create an array with 3 parts, the middle one being the text to be replaced
$var[1]=$new_text;
$my_string=implode('@@',$var);
echo $my_string;
?>