$gelensaat = '13:46';
$sonuc = date("H:i", strtotime("+30 minutes", strtotime($gelensaat)));
echo $sonuc;

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$gelensaat = '13:46';
$ekleneceksure = "30";
$sonuc = date("H:i", strtotime("+".$ekleneceksure." minutes", strtotime($gelensaat)));
echo $sonuc;