How Does PHP's ' ' Operator Merge Arrays and Handle Duplicate Keys?

Merging Arrays with the " " Operator: Unraveling its Behavior

In PHP, the operator facilitates the merging of two arrays, appending the elements of the right-hand array to the left-hand array. However, understanding how it handles duplicate keys is crucial.

How It Functions

According to the PHP Manual:

The + operator returns the right-hand array appended to the left-hand array; for keys that exist in both arrays, the elements from the left-hand array will be used, and the matching elements from the right-hand array will be ignored.

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"
}

Explanation

The operator appends the elements from the second array (test, oh) to the end of the first array (hi). However, it doesn't replace the duplicate key (hi), so it remains in the merged array.

Comparison with array_merge()

The operator differs from the array_merge() function in its behavior when handling duplicate keys. array_merge() overwrites duplicate keys from the left-hand array with keys from the right-hand array.

Implementation Details

The C-level implementation of the operator can be found in php-src/Zend/zend_operators.c. The logic is equivalent to the following snippet:

$union = $array1;

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

This snippet creates a new array ($union) based on the first array ($array1) and adds non-duplicate keys and values from the second array ($array2).

Conclusion

The operator in PHP provides a convenient way to merge arrays, but it's crucial to understand its specific behavior when it encounters duplicate keys. The array_merge() function offers an alternative that overwrites duplicate keys, allowing for more control over the merged array.

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