<form action='' method='post'>
    <table border="1">
        <tr>
            <td>Andrei</td>
            <td><?= generateOptionsTime("from"); ?></td>
            <td><?= generateOptionsTime("to"); ?></td>

        </tr>
        <tr>

        </tr>
    </table>
    <input type='submit'>
</form>

<?php

$difference = getTimeDifference();
if ($difference !== null) {
    echo "hours: " . $difference. "<br/>";
}

echo "<br/";

///////////////////////////////////////////////////////////////////////////////////

function getDayTypeTime($dayType){


    $difference = getTimeDifference();
    if ($difference !== null) {
        echo "hours: " . $difference. "<br/>";
    }

    return (int)$difference;
}


/////////////////////////////


function getTimeDifference()
{

    $from = getTimeFromPost("from");
    $to = getTimeFromPost("to");
    if ($from !== null && $to !== null) {

        return $to - $from;
    }
    return null;
}

function getTimeFromPost($type)
{
    $value = $_POST[$type];
    if ($value === null)
        return null;
    else
        return (int)$value;
}

function generateOptionsTime($type)
{
    $html = "<select name = $type>";
    for ($i = 0; $i <= 23; $i++) {
        $html .= "<option value ='$i'>" . date("H:i", strtotime("$i:00")) . "</option>";
    }
    $html .= "</select>";

    return $html;
}