If $time = midnight, change $day

If time is midnight 24:00+ $day must change to next day.. and $time reset.. (Sorry for my english please)

<?php

if ($time > "23") {
    $day + "1";
}
elseif ($time < "24") {
    $day + "0";
}

echo $day;

switch ($day) {
    case "1" : echo ("Mo"); break;
    case "2" : echo ("Th"); break;
    case "3" : echo ("We"); break;
    case "4" : echo ("Tu"); break;
    case "5" : echo ("Fr"); break;
    case "6" : echo ("Sa"); break;
    case "7" : echo ("Su"); break;
    default : echo ("-error-");
}

?>