php 38 lines · 8 steps

How memoization works in PHP closures

A higher-order function caches any callable's results by its arguments using a closure that captures state by reference.

Explained by highlit

Intermediate memoization closures higher-order-functions caching recursion

1function memoize(callable $fn): callable
2{
3    $cache = [];
4 
5    return function (...$args) use (&$cache, $fn) {
6        $key = md5(serialize($args));
7 
8        if (array_key_exists($key, $cache)) {
9            return $cache[$key];
10        }
11 
12        return $cache[$key] = $fn(...$args);
13    };
14}
15 
16$fibonacci = memoize(function (int $n) use (&$fibonacci): int {
17    return $n < 2 ? $n : $fibonacci($n - 1) + $fibonacci($n - 2);
18});
19 
20$primeFactors = memoize(function (int $number): array {
21    $factors = [];
22    $divisor = 2;
23 
24    while ($number > 1) {
25        while ($number % $divisor === 0) {
26            $factors[] = $divisor;
27            $number = intdiv($number, $divisor);
28        }
29        $divisor++;
30 
31        if ($divisor * $divisor > $number && $number > 1) {
32            $factors[] = $number;
33            break;
34        }
35    }
36 
37    return $factors;
38});

01 / 01

STEP 01

Walkthrough

Space play ←→ step click any line

Three takeaways

1Capturing a variable by reference lets a closure keep mutable state alive across many calls.
2Serializing and hashing arguments gives a stable cache key for arbitrary input shapes.
3Recursive functions gain the biggest speedup from memoization because each subproblem is solved once.

Related explainers

javascript

const ROLE_PERMISSIONS = {
  admin: ['users:read', 'users:write', 'billing:read', 'billing:write'],
  manager: ['users:read', 'billing:read'],
  member: ['users:read'],

Role-based permissions middleware in Express

authorization middleware rbac

Intermediate 9 steps

rust

use axum::{extract::{Path, State}, http::StatusCode, Json};
use dashmap::DashMap;
use serde::Serialize;
use std::sync::Arc;

Request coalescing in an Axum handler

caching concurrency request-coalescing

Advanced 8 steps

php

<?php
 
namespace App\Console\Commands;

Releasing stale document locks in Laravel

artisan-command transactions row-locking

Intermediate 6 steps

java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

Building a trie for autocomplete in Java

trie prefix-tree recursion

Intermediate 8 steps

php

<?php
 
namespace App\Providers;

Subdomain multi-tenancy routing in Laravel

multi-tenancy service-container route-binding

Advanced 7 steps

ruby

class KeyTransformer
  def self.camelize(data)
    new.camelize(data)
  end

Recursively camelizing nested Ruby data

recursion data-transformation pattern-matching

Intermediate 7 steps

Share this explainer

Here's the card — post it anywhere.

How memoization works in PHP closures — share card

Made with highlit — turn any snippet into a walkthrough like this in about a minute.

Explain your code