array_merge or + in PHP

PHP has several ways of combining two arrays into one. You can use array_merge or the + operator. There's a subtle difference between these two methods though, a difference worth knowing.

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Let's take a look at how these two methods compare:

array_merge($first, $second);

// vs.

$first + $second;

Let's say these are the two arrays we're working with:

$first = [
    'a',
    'b',
];

$second = [
    'c',
];

This would be the result of a simple array_merge call:

array_merge($first, $second);

[
    'a',
    'b',
    'c',
]

While the + operator gives us this result:

$first + $second;

[
    'a',
    'b',
]

Switching the operands while using the + operator, gives a different result:

$second + $first;

[
    'c',
    'b',
]

Confused? So was I.

Let's write out the $first and $second arrays in full, with their indices. This will make things more clear:

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

$second = [
    0 => 'c',
];

By now you can probably guess what's going on: the + operator will only add the elements of the rightside operand, if their key doesn't exist in the leftside operand, while array_merge will override existing keys.

By that definition, we can also determine that + can never be used to recursively merge arrays, as it will leave existing elements untouched:

$first = [
    'A' => [
        'B' => true,
        'C' => true,
    ],
];

$second = [
    'A' => [
        'B' => false,
        'C' => false,
    ],
];

$first + $second;

Here's the result:

[
    'A' => [
        'B' => true,
        'C' => true,
    ],
]

While using array_merge, would give this result:

[
    'A' => [
        'B' => false,
        'C' => false,
    ],
]

"Hang on", I hear you say, "isn't that what array_merge_recursive is supposed to do?".

Here we have a case of unfortunate naming. Please don't be surprised — it's PHP after all.

See, array_merge will merge matching elements by overriding them. array_merge_recursive on the other hand will keep both elements, and merge them in a new array, keeping both values.

This is what our previous example would look like, using array_merge_recursive:

[
    'A' => [
        'B' => [
            true,
            false,
        ],
        'C' => [
            true,
            false,
        ],
    ],
]

What about merging multiple arrays? You can probably guess the outcome by now:

$first = ['a'];
$second = ['b'];
$third = ['c'];

Here's what array_merge results in:

array_merge($first, $second, $third)

[
    'a',
    'b',
    'c',
]

Chaining the + operator also works, with the following result:

$first + $second + $third

[
    'a',
]

With this little refresher, I hope that you won't find yourself confused anymore when you're deep into your code and need to merge arrays.

I found it to be a cognitive burden when I had to stop and think about "hang on, what is the correct way to do this?". Luckily now, we know!