In PHP 5.3 there is a nice function that seems to do what I want:
strstr(input,\"\\n\",true)
Unfortunately, the server runs PHP 5.2.17 and the
list($line_1, $remaining) = explode("\n", $input, 2);
Makes it easy to get the top line and the content left behind if you wanted to repeat the operation. Otherwise use substr as suggested.