Using array_reduce() with a Callback to Reduce an Array to a Single Value

PHP's array_reduce() function applies a user‑defined callback to each element of an input array, cumulatively reducing the array to a single value.

The function signature is

mixed array_reduce (array $input, callable $function [, mixed $initial = NULL])

. The $input parameter is the array to be reduced, $function is the callback that receives the current accumulator and the current array element, and the optional $initial value serves as the initial accumulator (or as the return value when the array is empty).

When called, array_reduce() returns the final accumulator value; if the array is empty and no $initial is provided, it returns NULL.

Example usage:

<?php
function rsum($v, $w) {
    $v + $w;
    return $v;
}
function rmul($v, $w) {
    $v *= $w;
    return $v;
}
$a = array(1,2,3,4,5);
$x = array();
$b = array_reduce($a, "rsum");
$c = array_reduce($a, "rmul", 10);
$d = array_reduce($x, "rsum", "No data to reduce");
var_dump($a);
var_dump($b);
var_dump($c);
var_dump($d);
?>

The above code defines two callbacks: rsum (adds two numbers) and rmul (multiplies them). It then reduces the array $a with each callback, demonstrating the sum (result 15) and the product with an initial value of 10 (result 1200). It also shows how array_reduce() behaves with an empty array, returning the provided initial string.

Output of the var_dump calls is:

array(5) { [0]=> int(1) [1]=> int(2) [2]=> int(3) [3]=> int(4) [4]=> int(5) }
int(15)
int(1200)
string(17) "No data to reduce"