inserting a string after a certain line using php...

I have a text file as follows;

[context]
user=test;
user2=test2
password=secret
ipaddress: 192.x.x.x

I need to add new lines (host=ipaddress.com)after the string [context]. Can someone let me know how this can be done? I am trying to use someting line the following code, thank you.

A

 

<?php
    $fp = fopen('/file.txt', 'a+');
    fseek($fp, 2, SEEK_SET); 
    fwrite($fp, $newLine."\n");
    fclose($fp);
?>