How Does PHP's Array Union Operator ( ) Merge Arrays?

Array Union Operator ( ): Understanding Its Merging Behavior

Introduction

The " " operator, when applied to arrays in PHP, allows for the merging of data from two arrays. However, its behavior can be confusing as it doesn't always combine all elements. This article explores the mechanics of the " " operator and provides insights into how it merges arrays.

How the " " Operator Works

As stated in the PHP Language Operators Manual, the " " operator appends the elements of the right-hand array to the left-hand array. Crucially, for keys that appear in both arrays, the elements from the left-hand array take precedence and overwrite those from the right-hand array.

Example:

Consider the following example:

$test = array('hi');
$test += array('test', 'oh');
var_dump($test);

Output:

array(2) {
  [0]=>
  string(2) "hi"
  [1]=>
  string(2) "oh"
}

Here, the " " operator appends the elements from the right-hand array ('test' and 'oh') to the left-hand array ('hi'). However, since 'hi' is present in both arrays, the right-hand array value is ignored, and the left-hand array value is retained. As a result, the Output produces only two elements: 'hi' and 'oh', where 'oh' is added from the right-hand array.

Comparison with array_merge()

It's important to note that the behavior of the " " operator differs from the built-in array_merge() function. array_merge() combines all elements from both arrays, overwriting duplicate keys with the values from the right-hand array.

Implementation Details

The implementation logic of the " " operator is equivalent to the following snippet:

$union = $array1;

foreach ($array2 as $key => $value) {
    if (false === array_key_exists($key, $union)) {
        $union[$key] = $value;
    }
}

This logic ensures that the elements from the left-hand array have priority. Only elements with unique keys in the right-hand array are added to the union.

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