How Does PHP's ' ' Array Operator Differ from `array_merge()`?

Understanding Array Union Operation: ' ' in PHP

In PHP, the ' ' array operator allows for the merging of two arrays, combining their elements into a single resulting array. However, unlike other merging methods such as array_merge(), the ' ' operator has a specific behavior that can often lead to unexpected results.

How the ' ' Operator Works

According to the PHP Manual, the ' ' operator returns the right-hand array appended to the left-hand array. Crucially, for duplicate keys, the elements from the left-hand array take precedence, while those from the right-hand array are ignored.

Example

Consider the following example:

$array1 = ['one', 'two', 'foo' => 'bar'];
$array2 = ['three', 'four', 'five', 'foo' => 'baz'];

echo '<pre class="brush:php;toolbar:false">', print_r($array1 + $array2), '

';

Output:

Array
(
    [0] => one
    [1] => two
    [foo] => bar
    [2] => five
)

As seen in the output, the ' ' operator combined the arrays as follows:

• Elements with unique keys ('one', 'two', 'three', 'four', 'five') were preserved from both arrays.
• The duplicate key 'foo' resulted in the array element from the left-hand array ('bar') being used, overriding the value from the right-hand array ('baz').

Implementation Details

The ' ' operator's behavior is akin to the following logic:

$union = $array1;

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

Comparison with array_merge()

It's important to note that the behavior of ' ' differs from that of array_merge(). For example, using array_merge() on the same input would give the following output:

echo '<pre class="brush:php;toolbar:false">', print_r(array_merge($array1, $array2)), '

';

Output:

Array
(
    [0] => one
    [1] => two
    [foo] => baz
    [2] => three
    [3] => four
    [4] => five
)

In this case, array_merge() combines the arrays, with elements from the right-hand array overwriting those with duplicate keys from the left-hand array.

Conclusion

Understanding the nuanced behavior of the ' ' operator ensures that arrays are merged as desired in PHP code. Its preference for left-hand array elements over those from the right-hand array often leads to results that may be unexpected and require further scrutiny or alternative merging methods such as array_merge().

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