PHP Variable Scope

This article is created to cover the topic, scope of a variable in PHP.

Variable scope in PHP is one of the important topic, as we must know whether:

a variable defined outside a function can be accessed inside the function or not
a variable defined inside the function, can be accessed from outside the function or not
what to do, if we need to preserve the previous value of a variable defined inside a function
and so on

Before starting the tutorial, let me tell you, there are following three variable scopes available in PHP:

local
global
static

PHP Local Variable

Local variables in PHP are those variables that are defined within a function. And the scope of those variables are only to that particular function, where it is defined. For example:

<?php
   function codescracker()
   {
      $x = 500;
      echo $x;
   }
   codescracker();
?>

The output produced by above PHP example is shown in the snapshot given below:

The variable $x defined inside the function named codescracker(), is only accessible to inside the function. Now the question is, what if we try to access a local variable, from outside the function where it is defined?
Let's find out, using the example given below:

<?php
   function codescracker()
   {
      $x = 500;
   }
   echo $x;
?>

This time, the output produced by above example, is:

The similar output, you will get, if you will try to access a local variable from outside the scope where the variable is defined.

PHP Global Variable

Global variables in PHP are those variables that are defined outside a function. And the scope of those variables are to everywhere except function(s). For example:

<?php
   $x = 10;
   
   echo "<p>The value of \$x is $x</p><hr>";
   
   if($x>0)
      echo "<p>The value of \$x is greater than 0.</p><hr>";
   
   echo "<p>Printing the value of \$x for five times:</p>";
   for($i=0; $i<5; $i++)
      echo "$x<BR>";
?>

The output of above PHP example, is:

Now the question is, what if we will try to access a variable declared outside a function, from inside a function?
Let's find out, using an example given below:

<?php
   $x = 10;
   function codescracker()
   {
      echo $x;
   }
   codescracker();
?>

Now the output produced by this example, is:

You will get similar output every time, when you will try to access a variable from inside a function, that is defined outside the function. But PHP has a solution to use global variables from inside the function. The solution is, the global keyword, that is defined right after this paragraph.

PHP global Keyword

Use global keyword to declare a global variable inside a function to be able to access that global variable, in this way:

function functionName()
{
   global variableName;
}

That is, use global keyword to declare all the global variables, inside the function where you want to use. For example:

<?php
   $x = 10;
   $y = 20;
   function codescracker()
   {
      global $x;
      echo "<p>The value of \$x is $x</p>";
      global $y;
      echo "<p>The value of \$y is $y</p>";
      
      $z = $x + $y;
      echo "<p>\$x + \$y = $z</p>";
   }
   codescracker();
?>

The output of above PHP example, is:

However, PHP stores all variables that are declared globally, in an array named $GLOBALS[variable]. Therefore, we can use this array to access and update a global variable from within a function. For example:

<?php
   $x = 10;
   $y = 20;
   function codescracker()
   {
      echo "<p>The value of \$x is ", $GLOBALS['x'], "</p>";
      echo "<p>The value of \$y is ", $GLOBALS['y'], "</p>";
      
      $z = $GLOBALS['x'] + $GLOBALS['y'];
      echo "<p>\$x + \$y = $z</p>";
   }
   codescracker();
?>

You will get the same output as of previous example. Let me create another example, where I'm going to update the global variable, from within the function:

<?php
   $x = 10;
   echo "<p>\$x = $x (before function, before update)</p>";
   function codescracker()
   {
      echo "<p>\$x = ", $GLOBALS['x'], " (inside function, before update)</p>";
      $GLOBALS['x'] = 20;
      echo "<p>\$x = ", $GLOBALS['x'], " (inside function, after update)</p>";
   }
   codescracker();
   echo "<p>\$x = $x (after function, after update)</p>";
?>

Now the output of this PHP example, is:

PHP Static Variable

Static variables are those variables that preserves their previous value during the execution of a function. To create a static variable, we need to use the static keyword before the variable to make it static.

PHP static keyword comes into picture when we needed to execute a function for multiple times preserving or using its previous values. As normally, variables gets deleted after the execution of a function. For example:

<?php
   function codes()
   {
      $x = 100;
      echo "<p>$x</p>";
      $x++;
   }
   codes();
   codes();
   codes();
   codes();
   
   echo "<hr>";
   
   function cracker()
   {
      static $y = 200;
      echo "<p>$y</p>";
      $y++;
   }
   cracker();
   cracker();
   cracker();
   cracker();
?>

The output of above PHP example on static variable, is shown in the snapshot given below:

In above example, the variable $x inside the first function, that is codes() is defined as usual, therefore every time after executing the function, its value gets printed, and the variable gets deleted. But look at the variable $y defined in second function with static keyword, that makes the variable $y a static one. Therefore, it preserves its previous value, even after executing the function.

Here is another example, of static variable in PHP:

<?php
   function codescracker()
   {
      static $num = 2;
      echo "--Table of $num--<BR>";
      for($i=1; $i<=10; $i++)
         echo "$num * $i = ", $num*$i, "<BR>";
      $num++;
   }
   codescracker();
   echo "<HR>";
   codescracker();
?>

The snapshot given below shows the sample output produced by above PHP example: