Avoid using array_merge within a loop

July 4, 2025 · 2 mins  Found a typo? Edit me

If you're a PHP developer, chances are you've encountered (or written) code like this:

$userLists = [];
foreach ($this->someCall() as $someObject) {
    $userLists = array_merge($userLists, $someObject->getUsers());
}

The goal here is to populate the $userLists array with users returned from some external source (in this case, from $someObject->getUsers()). An instinct is to use array_merge inside the loop to combine arrays.

While this works, there's a performance caveat: array_merge creates a new array every time it's called, duplicating all elements and discarding the previous one. This approach may be fine for small arrays, but the cost grows significantly with larger datasets.

Alternatives for better performance

Spread operator (merge once at the end)

Instead of merging in every iteration, we collect all elements and merge them only once at the end.

$userLists = [];
foreach ($this->someCall() as $someObject) {
    $userLists[] = $someObject->getUsers();
}
$userLists = array_merge(...$userLists);

Pros and cons:

• 🟠 Requires PHP 7.4+ (spread operator for arrays)
• 🔴 If any $someObject->getUsers() returns an empty array, PHP emits a warning (you'll need to check or provide a default value)
• 🔴 Holds all intermediate arrays in memory before merging
• 🟢 Clean and modern syntax
• 🟢 Performs only one merge operation — faster than repeated array_merge() in a loop

Manual nested loop

This method avoids all intermediate arrays and merges on the fly.

$userLists = [];
foreach ($this->someCall() as $someObject) {
    foreach ($someObject->getUsers() as $user) {
        $userLists[] = $user;
    }
}

Pros and cons:

• 🟢 Works on all PHP versions
• 🟢 No risk of warning if $someObject->getUsers() returns an empty array (the inner loop is not executed)
• 🟢 Best memory performance for large datasets
• 🔴 Slightly more verbose