How to change a specific line of a file in php

I have a configuration file, and I would like to edit it by means of a page in php instead of going through the file manually, below the example.

$db['host'] = '127.0.0.1';
$db['port'] = '3306';
$db['user'] = "root";
$db['pass'] = 'root';
$db['db'] = "database";