Anonymous functions

Anonymous functions, also known as closures, allow the creation of functions which have no specified name. They are most useful as the value of callback parameters, but they have many other uses.

Example #1 Anonymous function example


<?php
echo preg_replace_callback('~-([a-z])~', function ($match) {
    return strtoupper($match[1]);
}, 'hello-world');
// outputs helloWorld
?>

Closures can also be used as the values of variables; PHP automatically converts such expressions into instances of the Closure internal class. Assigning a closure to a variable uses the same syntax as any other assignment, including the trailing semicolon:

Example #2 Anonymous function variable assignment example


<?php
$greet = function($name)
{
    printf("Hello %s\r\n", $name);
};

$greet('World');
$greet('PHP');
?>

Closures may also inherit variables from the parent scope. Any such variables must be passed to the use language construct.

Example #3 Inheriting variables from the parent scope


<?php
$message = 'hello';

// No "use"
$example = function () {
    var_dump($message);
};
echo $example();

// Inherit $message
$example = function () use ($message) {
    var_dump($message);
};
echo $example();

// Inherited variable's value is from when the function
// is defined, not when called
$message = 'world';
echo $example();

// Reset message
$message = 'hello';

// Inherit by-reference
$example = function () use (&$message) {
    var_dump($message);
};
echo $example();

// The changed value in the parent scope
// is reflected inside the function call
$message = 'world';
echo $example();

// Closures can also accept regular arguments
$example = function ($arg) use ($message) {
    var_dump($arg . ' ' . $message);
};
$example("hello");
?>

The above example will output something similar to:

Notice: Undefined variable: message in /example.php on line 6
NULL
string(5) "hello"
string(5) "hello"
string(5) "hello"
string(5) "world"
string(11) "hello world"

Inheriting variables from the parent scope is not the same as using global variables. Global variables exist in the global scope, which is the same no matter what function is executing. The parent scope of a closure is the function in which the closure was declared (not necessarily the function it was called from). See the following example:

Example #4 Closures and scoping


<?php
// A basic shopping cart which contains a list of added products
// and the quantity of each product. Includes a method which
// calculates the total price of the items in the cart using a
// closure as a callback.
class Cart
{
    const PRICE_BUTTER  = 1.00;
    const PRICE_MILK    = 3.00;
    const PRICE_EGGS    = 6.95;

    protected $products = array();
    
    public function add($product, $quantity)
    {
        $this->products[$product] = $quantity;
    }
    
    public function getQuantity($product)
    {
        return isset($this->products[$product]) ? $this->products[$product] :
               FALSE;
    }
    
    public function getTotal($tax)
    {
        $total = 0.00;
        
        $callback =
            function ($quantity, $product) use ($tax, &$total)
            {
                $pricePerItem = constant(__CLASS__ . "::PRICE_" .
                    strtoupper($product));
                $total += ($pricePerItem * $quantity) * ($tax + 1.0);
            };
        
        array_walk($this->products, $callback);
        return round($total, 2);
    }
}

$my_cart = new Cart;

// Add some items to the cart
$my_cart->add('butter', 1);
$my_cart->add('milk', 3);
$my_cart->add('eggs', 6);

// Print the total with a 5% sales tax.
print $my_cart->getTotal(0.05) . "\n";
// The result is 54.29
?>

Anonymous functions are implemented using the Closure class.

Changelog

Version	Description
5.4.0	`$this` can be used in anonymous functions.
5.3.0	Anonymous functions become available.

Notes

Note: It is possible to use func_num_args(), func_get_arg(), and func_get_args() from within a closure.