How Does the ' =' Operator Work with PHP Arrays, and Why Does It Behave Like a Union?

Union Operators in PHP Arrays: Unveiling the " =" Enigma

In PHP, arrays offer versatile manipulation options, and one intriguing operator is " =". This operator allows the merging of two arrays, but a common misconception arises from its apparent omission of certain elements. To shed light on this behavior, we delve into the inner workings of " =" and its unique approach to array combination.

When applied to arrays, " =" appends the elements of the right-hand array to the left-hand array. However, for duplicate keys, it favors the elements from the left-hand array, discarding their counterparts from the right-hand array. This behavior mimics a union operation, where only distinct elements are retained.

To illustrate, consider the following code:

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

The resulting array $test contains only two elements: "hi" and "oh". This occurs because "hi" is preserved from the left-hand array, and "test" is ignored due to the duplicate key.

To understand the technical underpinnings of " =", we turn to the PHP Language Operators documentation which states, "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."

Essentially, " =" operates by iterating over the right-hand array and adding any missing keys to the left-hand array. Keys that already exist in the left-hand array are not overwritten. This behavior differs from array_merge(), which combines arrays without regard to duplicate keys, resulting in a larger merged array.

For example:

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

$union = $array1 + $array2;

The resulting $union array would contain:

[
    'one' => 'one', // preserved from $array1
    'two' => 'two', // preserved from $array1
    'foo' => 'bar', // preserved from $array1
    'five' => 'five', // added from $array2
]

Understanding the nuanced behavior of " =" empowers developers to efficiently manipulate arrays, ensuring that their data remains intact and organized.

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