Update Table using PHP & SQL - php

Question

I am trying to update the table below timetable, I want to be able to input the day and subject to change via a selection box form and "add" another id (say 103) to ethics on tuesday at 2pm.
With my current code i am selecting the options in the selection form but it when submitted it returns a value of Ethics Ethics Ethics Ethics Ethics (5 times). Like so- 5x Returned Value
Also when i get into the page i get the following errors...
Notice: Undefined index: day in
/Applications/XAMPP/xamppfiles/htdocs/assignment/stuTimetable.php on
line 304
Notice: Undefined index: subject in
/Applications/XAMPP/xamppfiles/htdocs/assignment/stuTimetable.php on
line 305
Any help is greatly appreciated.....
PHP
$day = $_POST['day'];
$subject = $_POST['subject'];
$sqlevent = "SELECT '$subject' FROM timetable WHERE Day = '$day'";
$resultevent = mysql_query($sqlevent);
$row2 = mysql_fetch_row($resultevent);
$resultevent2 = $row2[0];
echo $resultevent2;
if($resultevent2 = true){
$sqlevent2 = "UPDATE timetable SET 9am = 'Hello' WHERE Day = '$day'";
}
<form action="<?php $_PHP_SELF ?>" id="showapp" method="POST">
<input name="submitapp" type="submit" value="Show Appointments">
</form>
<br>
<form action="<?php $_PHP_SELF ?>" id="timeForm" method="POST">
<h1>Book a slot:</h1>
<br>
Day:
<br>
<select name="day" form="timeForm">
<option value="">--Select--</option>
<option value="Monday">Monday</option>
<option value="Tuesday">Tuesday</option>
<option value="Wednesday">Wednesday</option>
<option value="Thursday">Thursday</option>
<option value="Friday">Friday</option>
</select>
<br>
Subject:
<br>
<select name="subject" form="timeForm">
<option value="">--Select--</option>
<option value="Meta-Physics">Meta-Physics</option>
<option value="Moral Theory">Moral Theory</option>
<option value="Ethics">Ethics</option>
<option value="Baking">Baking</option>
<option value="Appointment">Appointment</option>
</select>
<br>
Password:
<br>
<input type="password" name="password" placeholder="Type Password to Confirm Booking">
<br>
<input name="submit" type="submit" value="Submit">
</form>

Answer 1

You're getting this error because:
$day = $_POST['day'];
$subject = $_POST['subject'];
are executed once you load the page and at the first time these array elements are not defined.
It's only when you submit the form where $_POST is going hold the right values. So you have to check if you're coming from a form submit
if(isset($_POST['sumbit'])) {
// the update code here
}
Note that I've use submit. You usually have to check any required field because if the field is empty any element of the indexed array would use this field's name as and array key.

Answer 2

Check first if you are handling a post request using something like
if(isset($_POST['day'])) {
// the update code here
}