global variables in php not working as expected

问题:

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.

回答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.

回答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.

回答3:

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

回答4:

If you have a lot of variables you want to access during a task which uses many functions, consider making a 'context' object to hold the stuff:

//We're doing "foo", and we need importantString and relevantObject to do it
$fooContext = new StdClass(); //StdClass is an empty class
$fooContext->importantString = "a very important string";
$fooContext->relevantObject = new RelevantObject();

doFoo($fooContext);

Now just pass this object as a parameter to all the functions. You won't need global variables, and your function signatures stay clean. It's also easy to later replace the empty StdClass with a class that actually has relevant methods in it.

回答5:

The global scope spans included and required files, you don't need to use the global keyword unless using the variable from within a function. You could try using the $GLOBALS array instead.

回答6:

You must declare a variable as global before define values for it.

回答7:

It is useless till it is in the function or a class. Global means that you can use a variable in any part of program. So if the global is not contained in the function or a class there is no use of using Global