Variable scope

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- Variable scope

## Variable scope

The scope of a variable is the context within which it is defined. PHP has a function scope and a global scope. Any variable defined outside a function is limited to the global scope. When a file is included, the code it contains inherits the variable scope of the line on which the include occurs.

Example #1 Example of global variable scope

    <?php
    $a = 1;
    include 'b.inc'; // Variable $a will be available within b.inc
    ?>

Any variable created inside a named function or an anonymous function is limited to the scope of the function body. However, arrow functions bind variables from the parent scope to make them available inside the body. If a file include occurs inside a function within the calling file, the variables contained in the called file will be available as if they had been defined inside the calling function.

Example #2 Example of local variable scope

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

    function test()
    {
        echo $a; // Variable $a is undefined as it refers to a local version of $a
    }
    ?>

The example above will generate an undefined variable `E_WARNING` (or an `E_NOTICE` prior to PHP 8.0.0). This is because the echo statement refers to a local version of the $a variable, and it has not been assigned a value within this scope. Note 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

The `global` keyword is used to bind a variable from a global scope into a local scope. The keyword can be used with a list of variables or a single variable. A local variable will be created referencing the global variable of the same name. If the global variable does not exist, the variable will be created in global scope and assigned `null`.

Example #3 Using `global`

    <?php
    $a = 1;
    $b = 2;

    function Sum()
    {
        global $a, $b;

        $b = $a + $b;
    }

    Sum();
    echo $b;
    ?>

The above example 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 #4 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 #5 Example demonstrating superglobals and scope

    <?php
    function test_superglobal()
    {
        echo $_POST['name'];
    }
    ?>

> Note: Using `global` keyword outside a function is not an error. It can be used if the file is included from inside a function.

### 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 #6 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 #7 Example use of static variables

    <?php
    function test()
    {
        static $a = 0;
        echo $a;
        $a++;
    }
    ?>

Now, $a is initialized only in first call of function and 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. The following simple function recursively counts to 10, using the static variable $count to know when to stop:

Example #8 Static variables with recursive functions

    <?php
    function test()
    {
        static $count = 0;

        $count++;
        echo $count;
        if ($count < 10) {
            test();
        }
        $count--;
    }
    ?>

Prior to PHP 8.3.0, static variables could only be initialized using a constant expression. As of PHP 8.3.0, dynamic expressions (e.g. function calls) are also allowed:

Example #9 Declaring static variables

    <?php
    function foo(){
        static $int = 0;          // correct
        static $int = 1+2;        // correct
        static $int = sqrt(121);  // correct as of PHP 8.3.0

        $int++;
        echo $int;
    }
    ?>

Static variables inside anonymous functions persist only within that specific function instance. If the anonymous function is recreated on each call, the static variable will be reinitialized.

Example #10 Static variables in anonymous functions

    <?php
    function exampleFunction($input) {
        $result = (static function () use ($input) {
            static $counter = 0;
            $counter++;
            return "Input: $input, Counter: $counter\n";
        });

        return $result();
    }

    // Calls to exampleFunction will recreate the anonymous function, so the static
    // variable does not retain its value.
    echo exampleFunction('A'); // Outputs: Input: A, Counter: 1
    echo exampleFunction('B'); // Outputs: Input: B, Counter: 1
    ?>

As of PHP 8.1.0, when a method using static variables is inherited (but not overridden), the inherited method will now share static variables with the parent method. This means that static variables in methods now behave the same way as static properties.

As of PHP 8.3.0, static variables can be initialized with arbitrary expressions. This means that method calls, for example, can be used to initialize static variables.

Example #11 Usage of static Variables in Inherited Methods

    <?php
    class Foo {
        public static function counter() {
            static $counter = 0;
            $counter++;
            return $counter;
        }
    }
    class Bar extends Foo {}
    var_dump(Foo::counter()); // int(1)
    var_dump(Foo::counter()); // int(2)
    var_dump(Bar::counter()); // int(3), prior to PHP 8.1.0 int(1)
    var_dump(Bar::counter()); // int(4), prior to PHP 8.1.0 int(2)
    ?>

### References with `global` and `static` variables

PHP 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;
        $new = new stdClass;
        $obj = &$new;
    }

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

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

The above example will output:

    NULL
    object(stdClass)#1 (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)) {
            $new = new stdClass;
            // Assign a reference to the static variable
            $obj = &$new;
        }
        if (!isset($obj->property)) {
            $obj->property = 1;
        } else {
            $obj->property++;
        }
        return $obj;
    }

    function &get_instance_noref() {
        static $obj;

        echo 'Static object: ';
        var_dump($obj);
        if (!isset($obj)) {
            $new = new stdClass;
            // Assign the object to the static variable
            $obj = $new;
        }
        if (!isset($obj->property)) {
            $obj->property = 1;
        } else {
            $obj->property++;
        }
        return $obj;
    }

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

The above example will output:

    Static object: NULL
    Static object: NULL

    Static object: NULL
    Static object: object(stdClass)#3 (1) {
      ["property"]=>
      int(1)
    }

This example demonstrates that when assigning a reference to a static variable, it is not remembered when the `&get_instance_ref()` function is called a second time.