无法在PHP中添加时间

So I have this code:

$laikas = htmlspecialchars($_POST['rodymolaikas']);
$laikas = date( "H:i:s", strtotime( $laikas ) );

  $videotrukme = $_POST['videotrukme'];
  $videotrukme1 = round($videotrukme);
  $videotrukme2 = sprintf('%02d:%02d:%02d', ($videotrukme1/3600),($videotrukme/60%60), $videotrukme%60);

$laikas_end = date('H:i:s', strtotime($laikas) + strtotime($videotrukme2));

echo $laikas_end;

The variables are even to:

$laikas = 17:00:00;
$videotrukme2 = 00:01:12; 

But the answer I get is:

laikas_end = 02:00:00

Could you please tell me why is that happening?

You can accomplish time addition like thi

<?php
$laikas = htmlspecialchars($_POST['rodymolaikas']);
$laikas = date( "H:i:s", strtotime( $laikas ) );
$videotrukme2 = $_POST['rodymolaikas'];
$videotrukme2 = date( "H:i:s", strtotime( $videotrukme2 ) );

$h =  strtotime($laikas);
$h2 = strtotime($videotrukme2);

$minute = date("i", $h2);
$second = date("s", $h2);
$hour = date("H", $h2);

$convert = strtotime("+$minute minutes", $h);
$convert = strtotime("+$second seconds", $convert);
$convert = strtotime("+$hour hours", $convert);
$laikas_end = date('H:i:s', $convert);

echo $laikas_end;
?>