array_map / filter / reduce as FP Patterns

TL;DR

Explanation

Functional array operations: array_map(fn, array) — transforms each element, returns new array. array_filter(array, fn) — removes elements where fn returns false, preserves keys. array_reduce(array, fn, initial) — folds array to single value. Combining: array_values(array_filter(array_map(fn, $data))). PHP 8.1 arrow functions (fn($x) => $x*2) make these concise. Caveats: array_filter() preserves keys — use array_values() to re-index. array_map() with multiple arrays fills shorter ones with null. For large datasets, generators are more memory-efficient than array_map.

Common Misconception

Why It Matters

Common Mistakes

• Forgetting array_filter preserves keys — causes issues with JSON encoding expecting arrays.
• Using array_map when a generator would use less memory for large datasets.
• Nested array_map/filter — consider a single loop for readability beyond 2 levels.

Code Examples

$result = [];
foreach ($users as $user) {
    if ($user->isActive()) {
        $result[] = $user->getName();
    }
}

$result = array_map(
    fn(User $u) => $u->getName(),
    array_filter($users, fn(User $u) => $u->isActive())
);

// Sum with reduce:
$total = array_reduce($orders, fn($carry, $o) => $carry + $o->amount, 0);

References

• ↗ https://www.php.net/manual/en/function.array-map.php

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