Using PHP's array_replace_recursive() Function to Recursively Merge Arrays

PHP is a popular web programming language that provides a rich set of functions, including array_replace_recursive() , which can recursively merge two or more arrays by combining their keys, values, and sub‑arrays.

The basic syntax of the function is:

array_replace_recursive(array1, array2, array3......);

The function accepts multiple arrays as arguments and returns a new array that results from recursively merging the input arrays. When keys match, their values are merged; if a value is itself an array, the merge continues recursively until no nested arrays remain.

Example usage:

$array1 = array(
    'fruit' => array(
        'apple' => 1,
        'orange' => 4,
        'banana' => 3
    ),
    'vegetable' => array(
        'potato' => 2,
        'broccoli' => 1,
        'carrot' => 4
    )
);

$array2 = array(
    'fruit' => array(
        'orange' => 2
    ),
    'vegetable' => array(
        'potato' => 3,
        'broccoli' => 2,
        'carrot' => 1
    )
);

$result = array_replace_recursive($array1, $array2);
print_r($result);

The output of the above code is:

Array
(
    [fruit] => Array
        (
            [apple] => 1
            [orange] => 2
            [banana] => 3
        )

    [vegetable] => Array
        (
            [potato] => 3
            [broccoli] => 2
            [carrot] => 1
        )

)

As shown, the values from $array2 recursively overwrite the corresponding keys in $array1 , while keys that do not appear in the later arrays remain unchanged.

When using array_replace_recursive() , if the same key appears in multiple arrays, the later array's value overrides the earlier one. Keys must be strings or integers; otherwise a warning is generated.

If you need to keep existing keys and values while adding new ones from another array, you can use array_merge_recursive() . Unlike array_replace_recursive() , the merge function preserves both values for duplicate keys, creating nested arrays instead of overwriting.

In summary, array_replace_recursive() is a practical function for recursively merging arrays in PHP, allowing precise control over which keys are overwritten or retained, making it valuable for backend development tasks that involve complex array manipulation.