global variables in php not working as expected

Question

I'm having trouble with global variables in php. I have a $screen var set in one file, which requires another file that calls an initSession() defined in yet another file. The initSession() declares global $screen and then processes $screen further down using the value set in the very first script.

How is this possible?

To make things more confusing, if you try to set $screen again then call the initSession(), it uses the value first used once again. The following code will describe the process. Could someone have a go at explaining this?

$screen = "list1.inc";            // From model.php
require "controller.php";         // From model.php
initSession();                    // From controller.php
global $screen;                   // From Include.Session.inc  
echo $screen; // prints "list1.inc" // From anywhere
$screen = "delete1.inc";          // From model2.php
require "controller2.php"         
initSession();
global $screen;
echo $screen; // prints "list1.inc" 

Update:
If I declare $screen global again just before requiring the second model, $screen is updated properly for the initSession() method. Strange.

Answer 1

Global DOES NOT make the variable global. I know it's tricky :-)

Global says that a local variable will be used as if it was a variable with a higher scope.

E.G :

<?php  $var = "test"; // this is accessible in all the rest of the code, even an included one  function foo2() {     global $var;     echo $var; // this print "test"     $var = 'test2'; }  global $var; // this is totally useless, unless this file is included inside a class or function  function foo() {     echo $var; // this print nothing, you are using a local var     $var = 'test3'; }  foo(); foo2(); echo $var;  // this will print 'test2' ?> 

Note that global vars are rarely a good idea. You can code 99.99999% of the time without them and your code is much easier to maintain if you don't have fuzzy scopes. Avoid global if you can.

Answer 2

global $foo doesn't mean "make this variable global, so that everyone can use it". global $foo means "within the scope of this function, use the global variable $foo".

I am assuming from your example that each time, you are referring to $screen from within a function. If so you will need to use global $screen in each function.

Answer 3

You need to put "global $screen" in every function that references it, not just at the top of each file.