Using HTML variable in PHP in .php from Select - php

Question

Wow...all similar questions have serious downvotes. Guess I will be taking some as well, but been on this for a couple days and still stuck. So here it goes...
All of the following code is in index.php:
<form action="index.php" method="get">
<select name="month" id="month">
<option value="1">jan</option>
<option value="2">feb</option>
<option value="3">mar</option>
<option value="4">apr</option>
<option value="5">may</option>
<option value="6">jun</option>
<option value="7">jul</option>
<option value="8">aug</option>
<option value="9">sep</option>
<option value="10">oct</option>
<option value="11">nov</option>
<option value="12">dec</option>
</select>
</form>
<?php
$month = $_GET["month"];
echo "MONTH: $month";
$month in the php is NULL (Nothing prints out for $month). I want to be able to use it in the <?php ?> to make adjustments to it, then use it again in my HTML. How can I get the value from the HTML code for use in the PHP code?
I want to take 'value' and assign it to '$month'

Answer 1

Changes on the html page cannot change things within a php script as it has already executed on the server side. You need to setup some method to add the proper variable to the url (as you are using $_GET which is an array of the url variables in the query string, name value pairs after the ? in a url).
HTML
<select name="month" id="mySelect">
<option value="1">jan</option>
<option value="2">feb</option>
<option value="3">mar</option>
<option value="4">apr</option>
<option value="5">may</option>
<option value="6">jun</option>
<option value="7">jul</option>
<option value="8">aug</option>
<option value="9">sep</option>
<option value="10">oct</option>
<option value="11">nov</option>
<option value="12">dec</option>
</select>
JS
window.onload = function(){
var select = document.getElementById("mySelect");
select.onclick = function(){
var selectval = this.value;
window.location.href = "/somepage.php?month="+selectval;
};
};
Or using form submit, forms with the method GET will put all the input/select/textarea elements values into the url when submitted
<form action="somepage" method="GET">
<select name="month" id="mySelect">
<option value="1">jan</option>
<option value="2">feb</option>
<option value="3">mar</option>
<option value="4">apr</option>
<option value="5">may</option>
<option value="6">jun</option>
<option value="7">jul</option>
<option value="8">aug</option>
<option value="9">sep</option>
<option value="10">oct</option>
<option value="11">nov</option>
<option value="12">dec</option>
</select>
<input type="submit" name="submit" value="Submit">
</form>

Answer 2

PHP is a server side language. You cannot use php on client side. If you want to do something on client side, use JavaScript.
If you want to use php with your html either you have to refresh the page(see answer of Patrik Evans), or use Ajax.
If you are planning to use ajax as it is fast and simple, move your php to a different file like index-form.php and call it by ajax get method.
Basically when you load the page via browser, browser first lookup for your site IP from DNS. Then your server will compile the whole page (ignoring html, js and other client side script) and compile the server side scripts like PHP, Perl, Python etc depending upon the type of server. Then your server will send the compiled page to your browser and browser will read it, compile the client side scripts and render it in your viewport.