Memoization

Also Known As

TL;DR

Explanation

Memoization is an optimisation for pure (or effectively pure) functions where the same inputs always produce the same output. The first call computes the result and stores it in a lookup table; subsequent calls with the same arguments return the cached result instantly. In PHP, this is commonly implemented with a local static array or instance property. It is most valuable for recursive computations (Fibonacci, tree traversals), repeated regex compilations, or expensive lookups (translations, configuration values) called multiple times per request.

Common Misconception

Why It Matters

Common Mistakes

• Memoizing functions with side effects — the cached result bypasses the side effect on repeat calls.
• Not including all deterministic inputs in the cache key — different inputs produce the same key, returning wrong results.
• Unbounded memoization caches — memory grows indefinitely for functions called with many unique inputs.
• Memoizing across requests via static properties without TTL — stale results persist for the process lifetime.

Code Examples

function expensiveCalc(int $n): int {
  return // ... heavy computation each call
}

function expensiveCalc(int $n): int {
  static $cache = [];
  return $cache[$n] ??= /* heavy computation */;
}

References

• ↗ https://en.wikipedia.org/wiki/Memoization

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