PHP Variable Scopes

Summary: in this tutorial, you’ll learn about PHP variable scopes, which specify the part of code that can access a variable.

Introduction to PHP variable scopes

The scope of a variable determines which part of the code can access it. The locations where the variable can be accessible determine the scope of the variable.

In PHP, variables have four types of scopes:

  • Local
  • Global
  • Static
  • Function parameters

Local variables

When you define a variable inside a function, you can only access that variable within the function. And it’s said that the variable is local to the function.

The following example defines the say() function that displays the 'Hi' message:

<?php function say() { $message = 'Hi'; echo $message; }
Code language: PHP (php)

Inside the say() function, we define the $message variable. The $message variable is a local variable. And you cannot access it from the outside of the say() function.

Also, the $message variable only exists during the execution of the say() function. Once the say() function ends, the $mesage variable won’t exist anymore.

Global variables

When you declare a variable outside of a function, the variable is global. It means that you can access the variable anywhere within the script except inside a function. For example:

<?php $message = 'Hello'; function say() { $message = 'Hi'; echo $message; } echo $message; // Hello
Code language: PHP (php)

In this script, we have two variables with the same name $message.

The first variable is the global variable because we define it outside of a function. The $message variable that we define inside the function is the local variable. Even though these variables have the same name, they’re two different variables.

PHP allows you to access a global variable within a function by using the global keyword. For example:

<?php $message = 'Hello'; function say() { global $message; echo $message; // Hello } say();
Code language: PHP (php)

How it works.

  • First, define a global variable called $message.
  • Second, reference the global variable $message inside the say() function.

It’s important to note that it’s not a good practice to use global variables.

Superglobal variables

PHP has a list of built-in variables, which are known as superglobal variables. The superglobal variables provide information about the PHP script’s environment.

The superglobal variables are always available in all parts of the script. The following table shows the list of  PHP superglobal variables:

Superglobal VariablesMeaning
$GLOBALSReturns an array that contains global variables. The variable names are used to select which part of the array to access.
$_SERVERReturns data about the webserver environment.
$_GETReturn data from GET requests.
$_POSTReturn data from POST requests.
$_COOKIEReturn data from HTTP cookies
$_FILESReturn data from POST file uploads.
$_ENVReturn information about the script’s environment.
$_REQUESTReturn data from the HTTP request
$_SESSIONReturn variables registered in a session

Static variables

A static variable retains its value between function calls. Also, a static variable is only accessible inside the function. To define a static variable, you use the static keyword. For example:

<?php function get_counter() { static $counter = 1; return $counter++; } echo get_counter() . '<br>'; // 1 echo get_counter() . '<br>'; // 2 echo get_counter() . '<br>'; // 3
Code language: PHP (php)

Output:

1 2 3
Code language: plaintext (plaintext)

How it works.

  • First, define the get_counter() function with a static variable named $counter
  • Second, call the set_counter() function three times. As you notice that the value of the $counter variable is increased by one after each function call.

Function parameters

Function parameters are local to the function. Therefore, function parameters can only be accessible inside the function. For example:

<?php function sum($items) { $total = 0; foreach($items as $item) { $total += $item; } return $total; } // $items cannot be accessible here echo sum([10,20,30]);
Code language: PHP (php)

In this example, the $items is the parameter of the sum() function. It can only be accessible within the sum() function.

Summary

  • PHP has four types of variable scopes including local, global, static, and function parameters.
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