If you know the position of a substring in a given string, you can replace that substring by another string by using the substr_replace() function. Here is a PHP script on how to use substr_replace():

<?php

$string = "Warning: System will shutdown in NN minutes!";

$pos = strpos($string, "NN");

print(substr_replace($string, "15", $pos, 2)." ");

sleep(10*60);

print(substr_replace($string, "5", $pos, 2)." ");

?>

This script will print:

Warning: System will shutdown in 15 minutes!

(10 minutes later)

Warning: System will shutdown in 5 minutes!

Like substr(), substr_replace() can take negative starting position counted from the end of the string.