PHP - Variable Scope

In PHP, the scope of a variable is the context within which it is defined and accessible to the extent in which it is accessible. Generally, a simple sequential PHP script that doesn’t have any loop or a function etc., has a single scope. Any variable declared inside the "<?php" and "?>" tag is available throughout the program from the point of definition onwards.

Based on the scope, a PHP variable can be any of these three types −

A variable in a main script is also made available to any other script incorporated with include or require statements.

Example

In the following example, a "test.php" script is included in the main script.

main.php

<?php
   $var=100;
   include "test.php";
?>

test.php

<?php
   echo "value of \$var in test.php : " . $var;
?>

When the main script is executed, it will display the following output −

value of $var in test.php : 100

However, when the script has a user defined function, any variable inside has a local scope. As a result, a variable defined inside a function can't be accessed outside. Variables defined outside (above) the function have a global scope.

Example

Take a look at the following example −

<?php
   $var=100;   // global variable
   function myfunction() {
      $var1="Hello"; 	// local variable
      echo "var=$var  var1=$var1" . PHP_EOL;
   }
   myfunction();
   echo "var=$var  var1=$var1" . PHP_EOL; 
?>

It will produce the following output −

var=  var1=Hello
var=100  var1=

PHP Warning:  Undefined variable $var in /home/cg/root/64504/main.php on line 5
PHP Warning:  Undefined variable $var1 in /home/cg/root/64504/main.php on line 8

Note that a global variable is not automatically available within the local scope of a function. Also, the variable inside a function is not accessible outside.

The "global" Keyword

To enable access to a global variable inside local scope of a function, it should be explicitly done by using the "global" keyword.

Example

The PHP script is as follows −

<?php
   $a=10; 
   $b=20;
   echo "Global variables before function call: a = $a b = $b" . PHP_EOL;
   function myfunction() {
      global $a, $b;
      $c=($a+$b)/2;
      echo "inside function a = $a b = $b c = $c" . PHP_EOL;
      $a=$a+10;
   }
   myfunction();
   echo "Variables after function call: a = $a b = $b c = $c";
?>

It will produce the following output −

Global variables before function call: a = 10 b = 20
inside function a = 10 b = 20 c = 15
Variables after function call: a = 20 b = 20 c = 
PHP Warning:  Undefined variable $c in /home/cg/root/48499/main.php on line 12

Global variables can now be processed inside the function. Moreover, any changes made to the global variables inside the function will be reflected in the global namespace.

$GLOBALS Array

PHP stores all the global variables in an associative array called $GLOBALS. The name and value of the variables form the key-value pair.

Example

In the following PHP script, $GLOBALS array is used to access global variables −

<?php
   $a=10; 
   $b=20;
   echo "Global variables before function call: a = $a b = $b" . PHP_EOL;

   function myfunction() {
      $c=($GLOBALS['a']+$GLOBALS['b'])/2;
      echo "c = $c" . PHP_EOL;
      $GLOBALS['a']+=10;
   }
   myfunction();
   echo "Variables after function call: a = $a b = $b c = $c";
?>

It will produce the following output −

Global variables before function call: a = 10 b = 20
c = 15
PHP Warning:  Undefined variable $c in C:\xampp\htdocs\hello.php on line 12
Variables after function call: a = 20 b = 20 c =

Static Variable

A variable defined with static keyword is not initialized at every call to the function. Moreover, it retains its value of the previous call.

Example

Take a look at the following example −

<?php
   function myfunction() {
      static $x=0;
      echo "x = $x" . PHP_EOL;
      $x++;
   }
   for ($i=1; $i<=3; $i++) {
      echo "call to function :$i : ";
      myfunction();
   }
?>

It will produce the following output −

call to function :1 : x = 0
call to function :2 : x = 1
call to function :3 : x = 2

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