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PHP Global variable

We can illustrate this problem with the following pseudo-code

function foo() {
 global $bob;

Your first question here is an obvious one

Where did $bob come from?

Are you confused? Good. You've just learned why globals are confusing and considered a bad practice.

If this were a real program, your next bit of fun is to go track down all instances of $bob and hope you find the right one (this gets worse if $bob is used everywhere). Worse, if someone else goes and defines $bob (or you forgot and reused that variable) your code can break (in the above code example, having the wrong object, or no object at all, would cause a fatal error).

Since virtually all PHP programs make use of code like include('file.php'); your job maintaining code like this becomes exponentially harder the more files you add. Also, this makes the task of testing your applications very difficult. Suppose you use a global variable to hold your database connection:

$dbConnector = new DBConnector(...);
function doSomething() {
 global $dbConnector;

In order to unit test this function, you have to override the global $dbConnector variable, run the tests and then reset it to its original value, which is very bug prone:

function testSomething() {
 global $dbConnector;
 $bkp = $dbConnector; // Make backup
 $dbConnector = Mock::create('DBConnector'); // Override
 $dbConnector = $bkp; // Restore

Superglobal variables

Super globals in PHP are predefined variables, which are always available, can be accessed from any scope throughout the script.

There is no need to do global $variable; to access them within functions/methods, classes or files.

These PHP superglobal variables are listed below:

  • $_SERVER
  • $_POST
  • $_GET
  • $_FILES
  • $_ENV
  • $_COOKIE


In this page (written and validated by ) you learned about PHP Global variable . What's Next? If you are interested in completing PHP tutorial, your next topic will be learning about: PHP Variable Scope.

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