array_map

Let's assume we have following situation:
<?php
class MyFilterClass {
 public function filter(array $arr) {
 return array_map(function($value) {
 return $this->privateFilterMethod($value);
 });
 }
 private function privateFilterMethod($value) {
 if (is_numeric($value)) $value++;
 else $value .= '.';
 }
}
?>

This will work, because $this inside anonymous function (unlike for example javascript) is the instance of MyFilterClass inside which we called it.
I hope this would be useful for anyone.

To transpose rectangular two-dimension array, use the following code:
array_unshift($array, null);
$array = call_user_func_array("array_map", $array);
If you need to rotate rectangular two-dimension array on 90 degree, add the following line before or after (depending on the rotation direction you need) the code above:
$array = array_reverse($array);
Here is example:
<?php
$a = array(
 array(1, 2, 3),
 array(4, 5, 6));
array_unshift($a, null);
$a = call_user_func_array("array_map", $a);
print_r($a);
?>

Output:
Array
(
 [0] => Array
 (
 [0] => 1
 [1] => 4
 )
 [1] => Array
 (
 [0] => 2
 [1] => 5
 )
 [2] => Array
 (
 [0] => 3
 [1] => 6
 )
)

If the callback function to be called is a static method from a different namespace, the fully qualified method including namespace must be specified (a use statement is not sufficient to resolve the namespace of the callback function)
<?php
use MyTools;
$arr = [1, 2, 3];
$arr = array_map('Tools::myHelper', $arr);
?>

will cause TypeError: 
array_map() expects parameter 1 to be a valid callback, class 'Tools' not found.
Use the fully qualified name for the callback instead:
<?php
$arr = [1, 2, 3];
$arr = array_map('\MyTools\Tools::myHelper', $arr);
?>

You may be looking for a method to extract values of a multidimensional array on a conditional basis (i.e. a mixture between array_map and array_filter) other than a for/foreach loop. If so, you can take advantage of the fact that 1) the callback method on array_map returns null if no explicit return value is specified (as with everything else) and 2) array_filter with no arguments removes falsy values. 
So for example, provided you have:
<?php
$data = [
 [
 "name" => "John",
 "smoker" => false
 ],
 [
 "name" => "Mary",
 "smoker" => true
 ],
 [
 "name" => "Peter",
 "smoker" => false
 ],
 [
 "name" => "Tony",
 "smoker" => true
 ]
];
?>

You can extract the names of all the non-smokers with the following one-liner:
<?php
$names = array_filter(array_map(function($n) { if(!$n['smoker']) return $n['name']; }, $data));
?>

It's not necessarily better than a for/foreach loop, but the occasional one-liner for trivial tasks can help keep your code cleaner.

The most memory-efficient array_map_recursive().
<?php
function array_map_recursive(callable $func, array $arr) {
 array_walk_recursive($arr, function(&$v) use ($func) {
 $v = $func($v);
 });
 return $arr;
}
?>

If you want to pass an argument like ENT_QUOTES to htmlentities, you can do the follow.
<?php
$array = array_map( 'htmlentities' , $array, array_fill(0 , count($array) , ENT_QUOTES) );
?>

The third argument creates an equal sized array of $array filled with the parameter you want to give with your callback function.

/** 
 * Function which recursively applies a callback to all values and also its
 * keys, and returns the resulting array copy with the updated keys and 
 * values.
 * PHP's built-in function array_walk_recursive() only applies the passed 
 * callback to the array values, not the keys, so this function simply applies
 * the callback to the keys too (hence the need of working with a copy, 
 * as also updating the keys would lead to reference loss of the original
 * array). I needed something like this, hence my idea of sharing it here.
 *
 * @param callable $func callback which takes one parameter (value 
 * or key to be updated) and returns its 
 * updated value
 *
 * @param array $arr array of which keys and values shall be 
 * get updated 
 */
function array_map_recursive(
 callable $func,
 array $arr
) {
 // Initiate copied array which will hold all updated keys + values
 $result = [];
 // Iterate through the key-value pairs of the array
 foreach ( $arr as $key => $value ) {
 // Apply the callback to the key to create the updated key value
 $updated_key = $func( $key );
 // If the iterated value is not an array, that means we have reached the
 // deepest array level for the iterated key, so in that case, assign
 // the updated value to the updated key value in the final output array
 if ( ! is_array( $value ) ) {
 $result[$updated_key] = $func( $value );
 } else {
 // If the iterated value is an array, call the function recursively,
 // By taking the currently iterated value as the $arr argument
 $result[$updated_key] = array_map_recursive(
 $func,
 $arr[$key]
 );
 }
 } // end of iteration through k-v pairs
 // And at the very end, return the generated result set
 return $result;
 } // end of array_map_recursive() function definition

A general solution for the problem of wanting to know the keys in the callback, and/or retain the key association in the returned array:
<?php
/**
 * Like array_map() but callback also gets passed the current key as the
 * first argument like so:
 * function($key, $val, ...$vals) { ... }
 * ...and returned array always maintains key association, even if multiple
 * array arguments are passed.
 */
function array_map_assoc(callable $callback, array $array, array ...$arrays) {
 $keys = array_keys($array);
 array_unshift($arrays, $keys, $array);
 return array_combine($keys, array_map($callback, ...$arrays));
}
?>

Because it uses array_map() directly, it behaves the same way in regard to ignoring the keys of subsequent array arguments. It also has the same variadic signature.