How do you use anonymous functions in PHP?

Question

Hello,

Anonymous functions are available from PHP 5.3. Should I use them or avoid them? If so, how?

Thank you

Edited; just found some nice trick with php anonymous functions...

$container           = new DependencyInjectionContainer();
$container->mail     = function($container) {};
$conteiner->db       = function($container) {};
$container->memcache = function($container) {};

Answer 1

A typical use of anonymous functions is callback functions. For example, you could use them for callback from sort algorithms such as in function uksort (http://lv.php.net/uksort) or replacing algorithms such as preg_replace_callback (http://lv.php.net/manual/en/function.preg-replace-callback.php). Have not tried it myself in PHP, so this is just a guess.

Answer 2

Maybe you could just read PHP's article on Anonymous Functions. It's actually pretty good.

Answer 3

Anonymous functions are useful when using functions that require a callback function like array_filter or array_map do:

$arr = range(0, 10);
$arr_even = array_filter($arr, function($val) { return $val % 2 == 0; });
$arr_square = array_map(function($val) { return $val * $val; }, $arr);

Otherwise you would need to define a function that you possibly only use once:

function isEven($val) { return $val % 2 == 0; }
$arr_even = array_filter($arr, 'isEven');
function square($val) { return $val * $val; }
$arr_square = array_map('square', $arr);

Answer 4

Anonymous functions are available from PHP 5.3.

Anonymous functions have been available in PHP for a long time: create_function has been around since PHP 4.0.1. However you're quite right that there is a new concept and syntax available as of PHP 5.3.

Should I use them or avoid them? If so, how?

If you've ever used create_function before, then the new syntax can simply slip right in where you used that. As other answers have mentioned, a common case is for 'throwaway' functions where they are to be used just once (or in a single place at least). Commonly that comes in the form of callbacks for the likes of array_map/reduce/filter, preg_replace_callback, usort, etc..

Example of using anonymous functions to count the number of times letters appear in words (this could be done in a number of other ways, it is just an example):

$array = array('apple', 'banana', 'cherry', 'damson');

// For each item in the array, count the letters in the word
$array = array_map(function($value){
    $letters = str_split($value);
    $counts  = array_count_values($letters);
    return $counts;
}, $array);

// Sum the counts for each letter
$array = array_reduce($array, function($reduced, $value) {
    foreach ($value as $letter => $count) {
        if ( ! isset($reduced[$letter])) {
            $reduced[$letter] = 0;
        }
        $reduced[$letter] += $count;
    }
    return $reduced;
});

// Sort counts in descending order, no anonymous function here :-)
arsort($array);

print_r($array);

Which gives (snipped for brevity):

Array
(
    [a] => 5
    [n] => 3
    [e] => 2
    ... more ...
    [y] => 1
)