I am new to PHP and am taking the PHP course on W3Schools. In the form handling part of the course, I read that there are two methods of handling form data: POST and GET. I understand that depending on the method used, a superglobal variable is created which stores the data as key-value pairs. And at the destination page, the data is retrieved from this variable (using $_POST["key"] or $_GET["key"]).
But my question is: Where is the superglobal variable ($_POST or $_GET) stored? If it is on that same page on which the form exists, how can it be accessed by another destination page specified in the action attribute of the form tag?
Is it that the same set of these superglobal variables are accessible by all the pages on the server (contrary to my present belief that each page has its own set of those)?
The code below should make my question more clear:
<html>
<body>
<form action="welcome.php" method="post">
Name: <input type="text" name="name"><br>
E-mail: <input type="text" name="email"><br>
<input type="submit">
</form>
</body>
</html>
<html>
<body>
Welcome <?php echo $_POST["name"]; ?><br>
Your email address is: <?php echo $_POST["email"]; ?>
</body>
</html>
W3Schools explains as follows:
When the user fills out the form above and clicks the submit button, the form data is sent for processing to a PHP file named "welcome.php". The form data is sent with the HTTP POST method. To display the submitted data you could simply echo all the variables.
How do the variables $_POST["name"] and $_POST["email"] reach welcome.php?
When you send a request to a web server (i.e. apache, nginx), the request is parsed into pieces to be understood. Then, in PHP's case, the file requested is sent to the PHP interpreter with the data from the request.
In your example, you want a form to send a POST request to the page welcome.php. (If you're new to requests methods GET/POST/etc, then w3schools has a simple explanation on that: http://www.w3schools.com/tags/ref_httpmethods.asp) Your form has the method set to post, this can be get but it generally never is. If a user pressed a submit button within a tag, then the form sends to the request to the page set in action using the method set in method. Your form will send a POST request with the data name="..." and email="...".
When this request reaches PHP, all data sent to it will be stored in the method global variable. All post request place their data in $_POST, all get requests place their data in $_GET.
So when you submit your forms POST request, the page welcome.php will be requested by your browser (Firefox/Chrome) using the method POST and sending the data name and email in that request. On your server, PHP will open the file welcome.php and place the data into the $_POST array. Then, since $_POST is global, anywhere within welcome.php will have access to this variable.
If you manually go to the page welcome.php, you will be performing a GET request. Generally, only way to send a post request is through a form. So manually going to that page will have $_POST set to an empty array.
When I was learning PHP, I almost always had these lines at the top of the page so I knew what was going on:
<pre><?php print_r($_GET);?></pre>
<pre><?php print_r($_POST);?></pre>
This will print out the arrays in a pretty format, and the tags will make sure they're formatted correctly in the browser (since print_r uses newline characters, and browsers ignore those unless in pre tags).
You could also swap print_r with var_dump to get way more info about the variable.
This is a simple answer, there's way more to web technologies. And once you understand PHP enough I'd move into frameworks like CakePHP or CodeIgniter which takes away a lot of the little bits. It's important to understand the basics first though.