How to append file after string? php

Question

Ok lets say I have a File called File.txt and it contains

Hello
World
!!!!!

Ok now I have it so they may Post something to add to the string But it has ot be right under the string "Hello". How would I go about doing that ?

Answer 1

$mystring="string to append";
$file="file";
$data = file($file);
foreach ($data as $k=>$v){
    if ( strpos($v,"Hello") !==FALSE){
        $data[$k]=$data[$k] . "$mystring\n";
    }
}
file_put_contents($file,$data);