$filepathname = "./test.txt";
$target = "A line after which the code should be added";
$newline = "A new line";

$stats = file($filepathname, FILE_IGNORE_NEW_LINES);
$offset = array_search($target,$stats) +1;
array_splice($stats, $offset, 0, $newline);
file_put_contents($filepathname, join("n", $stats));

$dir = dirname(__FILE__);
glob_recursive($dir);
function glob_recursive($dir){
foreach(glob($dir."*", GLOB_NOSORT) as $filename){
if(is_dir($filename)) glob_recursive($filename."/*");
else{
foreach(glob($dir."test.txt", GLOB_NOSORT) as $filename){
echo $filename."<br>";
}
}
}
}