Never Use array_merge in a Loop

The spread operator to the rescue.

Flattening a one-level array

I have seen people using the array_merge function in a loop like:

$lists = [
[1, 2],
[3, 4],
[5, 6],
];
$merged = [];foreach($lists as $list) {
$merged = array_merge($merged, $list);
}
// $merged === [1, 2, 3, 4, 5, 6];

This is a very bad practice because it’s a (memory) performance killer!

Instead, you should use the spread operator (in PHP since 5.6!):

$lists = [
[1, 2],
[3, 4],
[5, 6],
];
$merged = array_merge(...$lists);
// === [1, 2, 3, 4, 5, 6];

Unpacking an assoc-array

What if you had an assoc-array instead like this one?

$lists = [
'key-1' => [1, 2],
'key-2' => [3, 4],
'key-3' => [5, 6],
];

In that case, you will need to unpack its values:

$merged = array_merge(...array_values($lists));
// === [1, 2, 3, 4, 5, 6];

In Functional Programming, this is known as flatting a list.

No loops & no more performance problem.

Flatting a multilevel array

What if you wanted to flat a multilevel array like this one?

$lists = [[1], 2, [[3, 4], 5], [[[]]], [[[6]]], 7, 8, []];

Or like this one, even with key-values?

$lists = [
'key-1' => [
1,
[2],
'key-2' => [
3,
[
'key-3' => [4, 5],
],
],
],
6,
'key-4' => [7, 8],
];

In these cases, you might want to use the internal standard library:

$merged = iterator_to_array(
new RecursiveIteratorIterator(
new RecursiveArrayIterator($lists)
),
$use_keys = false
);
// $merged === [1, 2, 3, 4, 5, 6, 7, 8];

Conclusion

Usually, being aware of how to flat a “2 level” array might be sufficient:

$flattenList = array_merge(...array_values($lists));

Otherwise, the internal standard library will help you deal with it.

Aka: Chema. I love writing about stuff that I find interesting and bring some value to my life, so I can share them with you. https://chemaclass.es