PHP global in functions

Globals are evil

This is true for the global keyword as well as everything else that reaches from a local scope to the global scope (statics, singletons, registries, constants). You do not want to use them. A function call should not have to rely on anything outside, e.g.

function fn()
{
    global $foo;              // never ever use that
    $a = SOME_CONSTANT        // do not use that
    $b = Foo::SOME_CONSTANT;  // do not use that unless self::
    $c = $GLOBALS['foo'];     // incl. any other superglobal ($_GET, …)
    $d = Foo::bar();          // any static call, incl. Singletons and Registries
}

All of these will make your code depend on the outside. Which means, you have to know the full global state your application is in before you can reliably call any of these. The function cannot exist without that environment.

Using the superglobals might not be an obvious flaw, but if you call your code from a Command Line, you don't have $_GET or $_POST. If your code relies on input from these, you are limiting yourself to a web environment. Just abstract the request into an object and use that instead.

In case of coupling hardcoded classnames (static, constants), your function also cannot exist without that class being available. That's less of an issue when it's classes from the same namespace, but when you start mix from different namespaces, you are creating a tangled mess.

Reuse is severly hampered by all of the above. So is unit-testing.

Also, your function signatures are lying when you couple to the global scope

function fn()

is a liar, because it claims I can call that function without passing anything to it. It is only when I look at the function body that I learn I have to set the environment into a certain state.

If your function requires arguments to run, make them explicit and pass them in:

function fn($arg1, $arg2)
{
    // do sth with $arguments
}

clearly conveys from the signature what it requires to be called. It is not dependent on the environment to be in a specific state. You dont have to do

$arg1 = 'foo';
$arg2 = 'bar';
fn();

It's a matter of pulling in (global keyword) vs pushing in (arguments). When you push in/inject dependencies, the function does not rely on the outside anymore. When you do fn(1) you dont have to have a variable holding 1 somewhere outside. But when you pull in global $one inside the function, you couple to the global scope and expect it to have a variable of that defined somewhere. The function is no longer independent then.

Even worse, when you are changing globals inside your function, your code will quickly be completely incomprehensible, because your functions are having sideeffects all over the place.

In lack of a better example, consider

function fn()
{
    global $foo;
    echo $foo;     // side effect: echo'ing
    $foo = 'bar';  // side effect: changing
}

And then you do

$foo = 'foo';
fn(); // prints foo
fn(); // prints bar <-- WTF!!

There is no way to see that $foo got changed from these three lines. Why would calling the same function with the same arguments all of a sudden change it's output or change a value in the global state? A function should do X for a defined input Y. Always.

This gets even more severe when using OOP, because OOP is about encapsulation and by reaching out to the global scope, you are breaking encapsulation. All these Singletons and Registries you see in frameworks are code smells that should be removed in favor of Dependency Injection. Decouple your code.

More Resources:

http://c2.com/cgi/wiki?GlobalVariablesAreBad
How is testing the registry pattern or singleton hard in PHP?
Flaw: Brittle Global State & Singletons
static considered harmful
Why Singletons have no use in PHP
SOLID (object-oriented design)

The one big reason against global is that it means the function is dependent on another scope. This will get messy very quickly.

$str1 = 'foo';
$str2 = 'bar';
$str3 = exampleConcat();

vs.

$str = exampleConcat('foo', 'bar');

Requiring $str1 and $str2 to be set up in the calling scope for the function to work means you introduce unnecessary dependencies. You can't rename these variables in this scope anymore without renaming them in the function as well, and thereby also in all other scopes you're using this function. This soon devolves into chaos as you're trying to keep track of your variable names.

global is a bad pattern even for including global things such as $db resources. There will come the day when you want to rename $db but can't, because your whole application depends on the name.

Limiting and separating the scope of variables is essential for writing any halfway complex application.

Globals are unavoidable.

It is an old discussion, but I still would like to add some thoughts because I miss them in the above mentioned answers. Those answers simplify what a global is too much and present solutions that are not at all solutions to the problem. The problem is: what is the proper way to deal with a global variable and the use of the keyword global? For that do we first have to examine and describe what a global is.

Take a look at this code of Zend - and please understand that I do not suggest that Zend is badly written:

class DecoratorPluginManager extends AbstractPluginManager
{
/**
 * Default set of decorators
 *
 * @var array
 */
protected $invokableClasses = array(
    'htmlcloud' => 'Zend\Tag\Cloud\Decorator\HtmlCloud',
    'htmltag'   => 'Zend\Tag\Cloud\Decorator\HtmlTag',
    'tag'       => 'Zend\Tag\Cloud\Decorator\HtmlTag',
   );

There are a lot of invisible dependencies here. Those constants are actually classes. You can also see require_once in some pages of this framework. Require_once is a global dependency, hence creating external dependencies. That is inevitable for a framework. How can you create a class like DecoratorPluginManager without a lot of external code on which it depends? It can not function without a lot of extras. Using the Zend framework, have you ever changed the implementation of an interface? An interface is in fact a global.

Another globally used application is Drupal. They are very concerned about proper design, but just like any big framework, they have a lot of external dependencies. Take a look at the globals in this page:

/**
 * @file
 * Initiates a browser-based installation of Drupal.
 */

/**
 * Root directory of Drupal installation.
 */
define('DRUPAL_ROOT', getcwd());

/**
 * Global flag to indicate that site is in installation mode.
 */
define('MAINTENANCE_MODE', 'install');

// Exit early if running an incompatible PHP version to avoid fatal errors.
if (version_compare(PHP_VERSION, '5.2.4') < 0) {
  print 'Your PHP installation is too old. Drupal requires at least PHP 5.2.4. See the     <a     href="http://drupal.org/requirements">system requirements</a> page for more     information.';
  exit;
}

// Start the installer.
require_once DRUPAL_ROOT . '/includes/install.core.inc';
install_drupal();

Ever written a redirect to the login page? That is changing a global value. (And then are you not saying 'WTF', which I consider as a good reaction to bad documentation of your application.) The problem with globals is not that they are globals, you need them in order to have a meaningful application. The problem is the complexity of the overall application which can make it a nightmare to handle. Sessions are globals, $_POST is a global, DRUPAL_ROOT is a global, the includes/install.core.inc' is an unmodifiable global. There is big world outside any function that is required in order to let that function do its job.

The answer of Gordon is incorrect, because he overrates the independence of a function and calling a function a liar is oversimplifying the situation. Functions do not lie and when you take a look at his example the function is designed improperly - his example is a bug. (By the way, I agree with this conclusion that one should decouple code.) The answer of deceze is not really a proper definition of the situation. Functions always function within a wider scope and his example is way too simplistic. We will all agree with him that that function is completely useless, because it returns a constant. That function is anyhow bad design. If you want to show that the practice is bad, please come with a relevant example. Renaming variables throughout an application is no big deal having a good IDE (or a tool). The question is about the scope of the variable, not the difference in scope with the function. There is a proper time for a function to perform its role in the process (that is why it is created in the first place) and at that proper time may it influence the functioning of the application as a whole, hence also working on global variables. The answer of xzyfer is a statement without argumentation. Globals are just as present in an application if you have procedural functions or OOP design. The next two ways of changing the value of a global are essentially the same:

function xzy($var){
 global $z;
 $z = $var;
}

function setZ($var){
 $this->z = $var;
}

In both instances is the value of $z changed within a specific function. In both ways of programming can you make those changes in a bunch of other places in the code. You could say that using global you could call $z anywhere and change there. Yes, you can. But will you? And when done in inapt places, should it then not be called a bug?

Bob Fanger comments on xzyfer.

Should anyone then just use anything and especially the keyword 'global'? No, but just like any type of design, try to analyze on what it depends and what depends on it. Try to find out when it changes and how it changes. Changing global values should only happen with those variables that can change with every request/response. That is, only to those variables that are belonging to the functional flow of a process, not to its technical implementation. The redirect of an URL to the login page belongs to the functional flow of a process, the implementation class used for an interface to the technical implementation. You can change the latter during the different versions of the application, but should not change those with every request/response.

To further understand when it is a problem working with globals and the keyword global and when not will I introduce the next sentence, which comes from Wim de Bie when writing about blogs: 'Personal yes, private no'. When a function is changing the value of a global variable in sake of its own functioning, then will I call that private use of a global variable and a bug. But when the change of the global variable is made for the proper processing of the application as a whole, like the redirect of the user to the login page, then is that in my opinion possibly good design, not by definition bad and certainly not an anti-pattern.

In retrospect to the answers of Gordon, deceze and xzyfer: they all have 'private yes'(and bugs) as examples. That is why they are opposed to the use of globals. I would do too. They, however, do not come with 'personal yes, private no'-examples like I have done in this answer several times.

Simply put there is rarely a reason to global and never a good one in modern PHP code IMHO. Especially if you're using PHP 5. And extra specially if you're develop Object Orientated code.

Globals negatively affect maintainability, readability and testability of code. Many uses of global can and should be replaced with Dependency Injection or simply passing the global object as a parameter.

function getCustomer($db, $id) {
    $row = $db->fetchRow('SELECT * FROM customer WHERE id = '.$db->quote($id));
    return $row;
}

PHP global in functions

Globals are evil

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