PHP Global variable

We can illustrate this problem with the following pseudo-code

function foo() {
 global $bob;
 $bob->doSomething();
}

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;
 $dbConnector->execute("...");
}

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
 assertTrue(foo());
 $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:

• $GLOBALS
• $_SERVER
• $_REQUEST
• $_POST
• $_GET
• $_FILES
• $_ENV
• $_COOKIE
• $_SESSION