function file_intersperse_contents($szFilename, $szAfter, $szInject)
{
    $szContent = file_get_contents($szFilename);
    $szContent = str_replace($szAfter, $szAfter . $szInject, $szContent);
    file_put_contents($szFilename, $szContent);
}

file_intersperse_contents
(
    './file.html',
    'gaga@eat.bullshit',
    "\ncustomer name: Tanax\ncustomer location: TalkPHP"

);