php如何读取下拉菜单选中数据

Question

<?php

$con = mysqli_connect($db_hostname,$db_username,$db_password,$db_database);
if (!$con) die("Unable to connect to MySQL: ".mysqli_connect_error());

$query = "select * from tutorials";
$result = mysqli_query($con,$query);
$result || die("Database access failed: ".mysqli_error($con));
$rows = mysqli_num_rows($result);
while ($row = mysqli_fetch_assoc($result)) {
    echo "Day: ", $row["day"],"<br />\n";
    echo "Time: ", $row["time1"],"<br />\n";
    echo "Available: ", $row["available2"],"<br />\n";
    echo "Time: ", $row["time2"],"<br />\n";
    echo "Available: ",$row["available"],"<br /><br />\n";
}

?>

Please choose a day from above timetable:

Select a day
<?php
$query = "select day from tutorials";
$result = mysqli_query($con,$query);
$result || die("Database access failed: ".mysqli_error($con));
while ($rows = mysqli_fetch_assoc($result)) {
?>

Please choose one time:

Select a time
<?php
if(isset($_POST['submit']))
$day = $_POST['selectDay'];
echo "The day is $day";
$query = "select time1 from tutorials where day='$day'";
$result = mysqli_query($con,$query);
$result || die("Database access failed: ".mysqli_error($con));
?>