Understanding how POST forms are processed in PHP

Question

This is more of an "understanding" question that a specific example. I'm coming from a Python/R/Scientific Computing background where I usually compile things or communicate through an interpreter. But everything is typically on the machine so there's no "communication" between server/client.

I'm now trying to learn PHP (in the hopes of letting users run my scripts from a web interface) and am curious what happens during a POST.

Consider the following script:

<!DOCTYPE html>
<html>
<head>
<title>Form Testing</title>
<meta charset="utf-8"/>
</head>
<body>

<?php
  if($_POST['formSubmit'] == "Submit")  {
    echo "Post Status is: ".$_POST['formSubmit']."\n";
    $varMovie = $_GET['formMovie'];
     echo "Your Favorite Movie Was: ".$varMovie;
  }else{
  echo "Post Status is: ".$_GET['formSubmit'] ."\n";

  }
?>

<form action="index.php" method="post">
    Which is your favorite movie?   
    <input type="text" name="formMovie" maxlength="50">
    <input type="submit" name="formSubmit" value="Submit">
</form>

</body>
</html>

I get that the Submit button sends the equivalent of python dictionary (associative array?) to the next page. Then the command $_POST['formSubmit'] pulls up the value.

But where is the value in between when I hit the submit button to when the page loads. In other words, after the sumbit button, what actually happens? Clearly, the page must create this associative array somewhere and pass it somehow but I'm not sure how it does it.

The idea with a get seems more clear. The URL is appended so the PHP engine can read the URL string and find the values of all the variables (I'm assuming that's what happens yes)?

Thank you for your help!

In brief, Apache (or other) populates it for you from the http request. You can observe that http request using any traffic logging program, including Chrome's dev tools. — Seth Battin, Feb 15 '14 at 02:25
You're using a POST method but tries to read it using `$_GET`, change those to `$_POST`. And yes it does create a associative array to either `$_GET` or `$_POST` depending on how your URL and form method is. — Prix, Feb 15 '14 at 02:46
@Prix If his form's method is set to `GET`, his code will work fine — Anid Monsur, Feb 15 '14 at 02:48
Here is a relevant question with answers about how $_POST parameters are submitted. http://stackoverflow.com/questions/4007969/application-x-www-form-urlencoded-or-multipart-form-data — Anid Monsur, Feb 15 '14 at 02:49
Hey, yeah the $GET was a relic of me playing with it to understand how both methods work. — user1357015, Feb 15 '14 at 02:53

score 4 · Accepted Answer · answered Feb 15 '14 at 02:57

HTTP GET passes all its data via the url in a query string:

http://example.com/script.php?key=value
                              ^^^^^^^^^^---GET query parameters

This could've been accomplished via:

<form method="GET" action="http://example.com/script.php"
   <input type="hidden" name="key" value="value" />
   <input type="submit" />
</form>

This URL becomes part of the HTTP request's headers:

GET /script.php?key=value HTTP/1.1
Host: example.com
Header1: value1
Header2: value2
Cookie: something=whatever

A GET request has no 'body' section - the entirety of the request is contained in the URL has the rest of the request's headers.

HTTP POST can also have query parameters if need be, but generally anything submitted via POST operation sends the data in the HTTP requests "body" section:

POST /script.php?key=value HTTP/1.1
Host: example.com
Header1: value1
Header2: value2
Cookie: something=whatever

foo=bar

And this would've been generated by the following form:

<form method="POST" action="/script.php?key=value" />
   <input type="hidden" name="foo" value="bar" />
   <input type="submit" />
</form>

Note the blank line between the headers and the key foo. That line is how the webserver knows when the the headers are done and the body begins.

For this particular sample POSt request, you'd end up with:

$_GET['key'] = 'value'; // from the requested URL
$_POST['foo'] = 'bar'; // from the request body

There's also $_REQUEST in PHP, which is an internal PHP-only construct that combines various sources, including $_GET and $_POST into a single monolithic data structure, but its data comes from the exactly same sources.

True 'nuff, but it's a silent part of the http spec, and not something that should be relied on to be functional — Marc B, Feb 15 '14 at 03:03

Understanding how POST forms are processed in PHP

1 Answers1