$numLines = 12;.$result = implode("\n", array_slice(explode("\n", $text), 0, $numLines));preg_match("~(?:[^\n]*(?:\n|$)){0,$numLines}~", $text, $match);
$result = $match[0];
// или
$result = preg_replace('~((?:[^\n]*\n){'.$numLines.'})[\s\S]*~', '$1', $text);