Why and how do you use anonymous functions in PHP?

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);

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)

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