Need help deciphering PHP code - php

Question

I am running through a jQuery Ajax tutorial here:
http://www.charlieperrins.com/2011/03/ajax-jquery-101/
Everything works perfectly but I have a question about this piece of code:
<?php if ($_POST['user']) : ?>
<?php
$user_id = $_POST['user'];
if (isset($db_data[$user_id])) {
$data = $db_data[$user_id];
} else {
echo 'Sorry, no user data matched your request - please try again';
die;
}
?>
I am most concerned with the very first line. What does that line do? I am trying to keep all the code in 1 set of php tags but I don't know how to do that. If I knew what the first line does, I might be able to figure it out. Any help is appreciated. I am trying to reverse engineer this to fit it into my app but can't do it without knowing what that top line does.
Thanks.

Answer 1

All this does is continues the if block until endif.
There is no endif, so nothing in this script runs unless there is data in $_POST['user'] that doesn't evaluate to false.
I would write this a bit differently:
<?php
if (isset($_POST['user'])) {
$user_id = $_POST['user'];
if (isset($db_data[$user_id])) {
$data = $db_data[$user_id];
} else {
echo 'Sorry, no user data matched your request - please try again';
die;
}
}
?>

Answer 2

The first line tests if the $_POST array has a key user, and that key contains a "truthy" (non-empty, among other things) value, indicating that a form was posted to this script. If no form data was posted, the rest of the script won't execute, such as if someone browsed directly to this PHP script without using the expected form to post to it. It is a technique often used when a form posts back to the same PHP script. Upon first arriving at the script, the $_POST will be empty. When the form is posted back to the same script, different actions can be taken when it contains values.
There need only be one <?php tag:
<?php
if ($_POST['user']) {
$user_id = $_POST['user'];
if (isset($db_data[$user_id])) {
$data = $db_data[$user_id];
} else {
echo 'Sorry, no user data matched your request - please try again';
die;
}
}
?>

Answer 3

This is Alternative syntax for control structures

Answer 4

<?php if ($_POST['user']) : ?> means if $_POST['user'] evaluates to true, execute the following code.
It can be compressed down to this:
<?php if ($_POST['user']) :
$user_id = $_POST['user'];
....
Also,
if ($_POST['user']) :
should be
if (isset($_POST['user']) && !empty(trim($_POST['user']))) :
That makes sure that $_POST['user'] has been set (generally $_POST contains variables from a form), and that it is not empty even with white-space removed.
See
Alternative syntax for control structures
$_POST
empty
trim

Answer 5

The if ($_POST['user']) line is saying this:
If the variable $_POST['user'] exists and is set to a non-false value.
The above condition fails if $_POST['user'] is 0, false, or '' (empty string).
It also isn't safely checking that value.
You are better off using:
if (isset($_POST['user'])) && $_POST['user'] != '')
This way no warning is output when PHP has display_errors and notices turned on.