PHP array_merge vs array_replace vs + Union: The Definitive Guide

In 2016, I answered a question on Stack Overflow in Portuguese about the difference between array_merge, array_replace, and the + (union) operator in PHP. It scored 10 upvotes, and the confusion it addressed was real — PHP has three ways to combine arrays, and each handles duplicate keys and numeric indexes differently.

The 2016 Answer: Three Functions, Three Behaviors

array_merge — Reindexes Numeric Keys

$a = [0 => 'a', 1 => 'b'];
$b = [0 => 'c', 1 => 'd'];

array_merge($a, $b);
// [0 => 'a', 1 => 'b', 2 => 'c', 3 => 'd']

For numeric keys, array_merge reindexes everything sequentially. For string keys, the second array’s values overwrite the first’s:

$a = ['name' => 'Gabriel', 'role' => 'dev'];
$b = ['name' => 'Updated', 'city' => 'SP'];

array_merge($a, $b);
// ['name' => 'Updated', 'role' => 'dev', 'city' => 'SP']

array_replace — Overwrites by Key Position

$a = [0 => 'a', 1 => 'b'];
$b = [0 => 'c', 1 => 'd'];

array_replace($a, $b);
// [0 => 'c', 1 => 'd']

Unlike array_merge, it preserves numeric key positions and overwrites values at the same index. Works the same for string keys — second array wins.

+ (Union) — First Array Wins

$a = ['name' => 'Gabriel', 'role' => 'dev'];
$b = ['name' => 'Other', 'city' => 'SP'];

$a + $b;
// ['name' => 'Gabriel', 'role' => 'dev', 'city' => 'SP']

The union operator keeps the first array’s values for duplicate keys. It only adds keys that don’t already exist. This is the opposite of array_merge for string keys.

The 2026 Update: Spread Operator

PHP 7.4+ introduced the spread operator for arrays, and PHP 8.1+ extended it to string keys:

// PHP 8.1+: Spread with string keys (last wins)
$defaults = ['theme' => 'dark', 'lang' => 'en', 'debug' => false];
$overrides = ['theme' => 'light', 'debug' => true];

$config = [...$defaults, ...$overrides];
// ['theme' => 'light', 'lang' => 'en', 'debug' => true]

This is equivalent to array_merge but reads more naturally, especially for configuration merging. It’s the pattern most PHP developers reach for in 2026.

When to Use Each

Function	Numeric keys	String keys	Use case
array_merge	Reindexes	Last wins	Concatenating lists
array_replace	Preserves position	Last wins	Updating specific indexes
+ (union)	First wins	First wins	Applying defaults
[...$a, ...$b]	Reindexes	Last wins	Config merging (modern)

The Defaults Pattern

The most common real-world use case is merging user options with defaults. Here the union operator shines:

function createWidget(array $options = []) {
    $defaults = ['width' => 100, 'height' => 50, 'color' => 'blue'];
    $config = $options + $defaults; // User values win, defaults fill gaps
    // ...
}

Or with the spread operator (last-wins semantics, so defaults go first):

function createWidget(array $options = []) {
    $config = ['width' => 100, 'height' => 50, 'color' => 'blue', ...$options];
    // ...
}

Key Takeaway

PHP’s array combining functions aren’t interchangeable — they differ in how they handle duplicate keys and numeric indexes. In 2026, the spread operator covers 90% of cases and reads more clearly than function calls. But understanding + for defaults and array_replace for positional updates still matters when you hit the edge cases.