PHP - Global Variables

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In PHP, any variable that can be accessed from anywhere in a PHP script is called as a global variable. If the variable is declared outside all the functions or classes in the script, it becomes a global variable.

While global variables can be accessed directly outside a function, they aren’t automatically available inside a function.

Example

In the script below, $name is global for the function sayhello().

<?php
   $name = "Amar";
   function sayhello() {
      echo "Hello " . $name;
   }
   sayhello();
?>

However, the variable is not accessible inside the function. Hence, you will get an error message "Undefined variable $name".

Hello 
PHP Warning: Undefined variable $name in /home/cg/root/93427/main.php on line 5

Example

To get access within a function, you need to use the "global" keyword before the variable.

<?php
   $name = "Amar";
   function sayhello() {
      GLOBAL $name;
      echo "Hello " . $name;
   }
   sayhello();
?>

It will produce the following output −

Hello Amar

If a function accesses a global variable and modifies it, the modified value is available everywhere after the function call is completed.

Let us change the value of $name inside the sayhello() function and check its value after the function is called.

Example

Take a look at this following example −

<?php
   $name = "Amar";
   function sayhello() {
      GLOBAL $name;
      echo "Global variable name: $name" .PHP_EOL;
      $name = "Amarjyot";
      echo "Global variable name changed to: $name" .PHP_EOL;
   }
   sayhello();
   echo "Global variable name after function call: $name" .PHP_EOL;
?>

It will produce the following output −

Global variable name: Amar
Global variable name changed to: Amarjyot
Global variable name after function call: Amarjyot

The $GLOBALS Array

PHP maintains an associative array named $GLOBALS that holds all the variables and their values declared in a global scope. The $GLOBALS array also stores many predefined variables called as superglobals, along with the user defined global variables.

Any of the global variables can also be accessed inside any function with the help of a regular syntax of accessing an arrow element. For example, the value of the global variable $name is given by $GLOBALS["name"].

Example

In the following example, two global variable $x and $y are accessed inside the addition() function.

<?php
   $x = 10;
   $y = 20;

   function addition() {
      $z = $GLOBALS['x']+$GLOBALS['y'];
      echo "Addition: $z" .PHP_EOL;
   }
   addition();
?>

It will produce the following output −

Addition: 30

Example

You can also add any local variable into the global scope by adding it in the $GLOBALS array. Let us add $z in the global scope.

<?php
   $x = 10;
   $y = 20;
   function addition() {
      $z = $GLOBALS['x']+$GLOBALS['y'];
      $GLOBALS['z'] = $z;
   }
   addition();
   echo "Now z is the global variable. Addition: $z" .PHP_EOL;
?>

It will produce the following output −

Now z is the global variable. Addition: 30

Including One PHP Script in Another

You can include one PHP script in another. Variables declared in the included script are added in the global scope of the PHP script in which it is included.

Here is "a.php" file −

<?php
   include 'b.php';
   function addition() {
      $z = $GLOBALS['x']+$GLOBALS['y'];
      echo "Addition: $z" .PHP_EOL;
   }
   addition();
?>

It includes "b.php" that has the $x and $y variables, so they become the global variables for the addition() function in "a.php" script.

<?php
   $x = 10;
   $y = 20;
?>

Global variables are generally used while implementing singleton patterns, and accessing registers in embedded systems and also when a variable is being used by many functions.