Variable scope

The scope of a variable is the context within which it is defined. For the most part all PHP variables only have a single scope. This single scope spans included and required files as well. For example:


<?php
$a = 1;
include 'b.inc';
?>

Here the $a variable will be available within the included b.inc script. However, within user-defined functions a local function scope is introduced. Any variable used inside a function is by default limited to the local function scope. For example:


<?php
$a = 1; /* global scope */ 

function Test()
{ 
    echo $a; /* reference to local scope variable */ 
} 

Test();
?>

This script will not produce any output because the echo statement refers to a local version of the $a variable, and it has not been assigned a value within this scope. You may notice that this is a little bit different from the C language in that global variables in C are automatically available to functions unless specifically overridden by a local definition. This can cause some problems in that people may inadvertently change a global variable. In PHP global variables must be declared global inside a function if they are going to be used in that function.

The global keyword

First, an example use of global:

Example 12-2. Using global
<?php $a = 1; $b = 2; function Sum() { global $a, $b; $b = $a + $b; } Sum(); echo $b; ?>

The above script will output "3". By declaring $a and $b global within the function, all references to either variable will refer to the global version. There is no limit to the number of global variables that can be manipulated by a function.

A second way to access variables from the global scope is to use the special PHP-defined $GLOBALS array. The previous example can be rewritten as:

Example 12-3. Using $GLOBALS instead of global
<?php $a = 1; $b = 2; function Sum() { $GLOBALS['b'] = $GLOBALS['a'] + $GLOBALS['b']; } Sum(); echo $b; ?>

The $GLOBALS array is an associative array with the name of the global variable being the key and the contents of that variable being the value of the array element. Notice how $GLOBALS exists in any scope, this is because $GLOBALS is a superglobal. Here's an example demonstrating the power of superglobals:

Example 12-4. Example demonstrating superglobals and scope
<?php function test_global() { // Most predefined variables aren't "super" and require // 'global' to be available to the functions local scope. global $HTTP_POST_VARS; echo $HTTP_POST_VARS['name']; // Superglobals are available in any scope and do // not require 'global'. Superglobals are available // as of PHP 4.1.0, and HTTP_POST_VARS is now // deemed deprecated. echo $_POST['name']; } ?>

Using static variables

Another important feature of variable scoping is the static variable. A static variable exists only in a local function scope, but it does not lose its value when program execution leaves this scope. Consider the following example:

Example 12-5. Example demonstrating need for static variables
<?php function Test() { $a = 0; echo $a; $a++; } ?>

This function is quite useless since every time it is called it sets $a to 0 and prints "0". The $a++ which increments the variable serves no purpose since as soon as the function exits the $a variable disappears. To make a useful counting function which will not lose track of the current count, the $a variable is declared static:

Example 12-6. Example use of static variables
<?php function Test() { static $a = 0; echo $a; $a++; } ?>

Now, every time the Test() function is called it will print the value of $a and increment it.

Static variables also provide one way to deal with recursive functions. A recursive function is one which calls itself. Care must be taken when writing a recursive function because it is possible to make it recurse indefinitely. You must make sure you have an adequate way of terminating the recursion. The following simple function recursively counts to 10, using the static variable $count to know when to stop:

Example 12-7. Static variables with recursive functions
<?php function Test() { static $count = 0; $count++; echo $count; if ($count < 10) { Test(); } $count--; } ?>

Note: Static variables may be declared as seen in the examples above. Trying to assign values to these variables which are the result of expressions will cause a parse error.

Example 12-8. Declaring static variables
<?php function foo(){ static $int = 0; // correct static $int = 1+2; // wrong (as it is an expression) static $int = sqrt(121); // wrong (as it is an expression too) $int++; echo $int; } ?>

References with global and static variables

The Zend Engine 1, driving PHP 4, implements the static and global modifier for variables in terms of references. For example, a true global variable imported inside a function scope with the global statement actually creates a reference to the global variable. This can lead to unexpected behaviour which the following example addresses:


<?php
function test_global_ref() {
    global $obj;
    $obj = &new stdclass;
}

function test_global_noref() {
    global $obj;
    $obj = new stdclass;
}

test_global_ref();
var_dump($obj);
test_global_noref();
var_dump($obj);
?>

Executing this example will result in the following output:

NULL
object(stdClass)(0) {
}

A similar behaviour applies to the static statement. References are not stored statically:


<?php
function &get_instance_ref() {
    static $obj;

    echo 'Static object: ';
    var_dump($obj);
    if (!isset($obj)) {
        // Assign a reference to the static variable
        $obj = &new stdclass;
    }
    $obj->property++;
    return $obj;
}

function &get_instance_noref() {
    static $obj;

    echo 'Static object: ';
    var_dump($obj);
    if (!isset($obj)) {
        // Assign the object to the static variable
        $obj = new stdclass;
    }
    $obj->property++;
    return $obj;
}

$obj1 = get_instance_ref();
$still_obj1 = get_instance_ref();
echo "\n";
$obj2 = get_instance_noref();
$still_obj2 = get_instance_noref();
?>

Executing this example will result in the following output:

Static object: NULL
Static object: NULL

Static object: NULL
Static object: object(stdClass)(1) {
  ["property"]=>
  int(1)
}

This example demonstrates that when assigning a reference to a static variable, it's not remembered when you call the &get_instance_ref() function a second time.