PHP Manual
Performance optimization

Merging large arrays in PHP

06. 02. 2020

Often we need to merge multiple arrays together, this can be done very elegantly with the `array_merge` function:

$userIdsA = [1, 2, 3];
$userIdsB = [5, 6, 7];
// returns [1, 2, 3, 5, 6, 7]
$finalIds = array_merge($userIdsA, $userIdsB);

The array_merge function combines two arrays into one large array. If there is a collision in the keys, the value of the rightmost array wins.

Repeated merging in a loop

However, we often get an array of arrays that is created only in a loop (for example, from a database and then passed through foreach), and so we don't know the number of merges in advance.

A naive solution might look like this:

$finalIds = [];
foreach ($users as $user) {
$finalIds = array_merge($finalIds, $user->someIds);

However, this solution is very CPU inefficient because we have to merge the arrays together with each iteration and iterate over the entire large array.

However, there is a simple solution where we modify the merging algorithm to only go through the data once:

$finalIds = [];
foreach ($users as $user) {
$finalIds[] = $user->someIds;

In this case, the $finalIds array will generate a bit more data, but it's still less of a problem than the time-saving benefit.

The merging itself varies depending on the version of PHP you are using and is solved with an elegant trick:

/* PHP 5.6 and earlier */
$finalIds = call_user_func_array('array_merge', $finalIds + [[]]);
/* PHP 5.6+ and later */
$finalIds = array_merge([], ...$finalIds);
/* PHP 7.4+ and later for non-empty arrays */
$finalIds = array_merge(...$finalIds);

In particular, the array_merge(...$finalIds) solution looks very interesting, as it takes advantage of a new PHP 7 concept where you can pass a dynamic number of arguments to a function using the triplet character at the beginning. The merge process is then as efficient as possible and the whole logic is handled automatically by PHP internally.

The shorthand notation array_merge(...$finalIds) can only be used for non-empty arrays. If it is an empty array, no argument is passed to the function and the function throws a Function array_merge invoked with 0 parameters, at least 1 required. error.

Jan Barášek   Více o autorovi

Autor článku pracuje jako seniorní vývojář a software architekt v Praze. Navrhuje a spravuje velké webové aplikace, které znáte a používáte. Od roku 2009 nabral bohaté zkušenosti, které tímto webem předává dál.

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