Then you probably want to use 4-arg substr to truncate the buffer and save the fragment all at once:

$end = rindex($buffer, "\n");
if ($end < 0) { whoops, incomplete line }
$fragment = substr($buffer, $end+1, length($buffer), '');
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Note that length($buffer) is bigger than the actual length(buffer)-$end-1 you want, but specifying a length larger than is available is guaranteed to not cause problems; you'll just get what is there. If the buffer ends with "\n", you'll get an empty $fragment.