Spread Operator in Arrays [...$a, ...$b]

TL;DR

Explanation

PHP 7.4 added spread in function calls. PHP 8.1 added spread in array expressions: [...$a, ...$b] creates a new merged array. Key behavior: integer keys are re-indexed (like array_merge), string keys from later arrays overwrite earlier ones (like array_merge). PHP 8.1 also allows string-keyed spreads. Spread preserves no reference — it creates copies. For associative merging with last-write-wins semantics, [...$defaults, ...$overrides] is idiomatic.

Common Misconception

Why It Matters

Common Mistakes

• Expecting integer keys to be preserved — they're re-indexed.
• Using spread with string keys before PHP 8.1 — fatal error in PHP 7.4 spread.
• Not knowing that [...$a, ...$b] and array_merge($a, $b) behave identically for simple cases.

Code Examples

$merged = array_merge($defaults, $userConfig, ['debug' => false]);
// Works but verbose

// PHP 8.1+ idiomatic:
$config = [...$defaults, ...$userConfig, 'debug' => false];

// Cloning with overrides:
$updated = [...$original, 'status' => 'active', 'updated_at' => now()];

// Conditional spread:
$params = [
    'limit' => 20,
    ...($search ? ['q' => $search] : []),
];

References

• ↗ https://www.php.net/manual/en/language.types.array.php

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