Mastering PHP Array Merging: Using the + Operator

When working with PHP, one of the most common tasks is merging arrays. While many developers may be familiar with the array_merge() function, another powerful and simpler option is using the + operator. In this article, we'll explore how this operator works for array merging, its nuances, and the key considerations to keep in mind to make the most of this functionality.

The Basics: What Does the + Operator Do?

The + operator in PHP is used for array union, which combines two arrays into a new one. It adds the elements of the second array to the first, but only if the keys are not already present in the first array. This method does not overwrite existing values; it preserves the values from the first array if the keys are shared.

Example Code

Let's break down an example of how the + operator works for merging arrays:

$first = ['item1' => 'value1', 'item2' => 'value2'];
$second = ['item3' => 'value3', 'item4' => 'value4'];

$merged = $first + $second;

var_dump($merged);

Expected Output

The output of the code above will look like this:

array(4) {
  ["item1"]=> string(6) "value1"
  ["item2"]=> string(6) "value2"
  ["item3"]=> string(6) "value3"
  ["item4"]=> string(6) "value4"
}

Explanation of the Output

• The keys item1 and item2 are present in both the first array and the merged array with their original values.
• The keys item3 and item4 come from the second array and are added to the merged array because they do not exist in the first array.

Important Considerations

1. Key Overwriting: The + operator does not overwrite existing keys. For instance:

$first = ['item1' => 'value1'];
$second = ['item1' => 'value2', 'item2' => 'value3'];

$merged = $first + $second;

var_dump($merged);

Output:

array(2) {
  ["item1"]=> string(6) "value1"
  ["item2"]=> string(6) "value3"
}

The value of item1 from the first array is retained, while item2 is added from the second array.

2. Preservation of Order: The order of the arrays can affect how elements appear in the final merged array. The + operator will add the second array’s elements only if their keys don’t exist in the first. This means that the order of arrays matters.
3. Array Keys: When merging arrays using the + operator, keys are not reindexed. This ensures that if you're merging arrays with numerical keys, you won't have a reshuffled array where elements are indexed starting from 0.

Using array_merge() for Comparison

It's important to note that array_merge() behaves differently from the + operator:

• array_merge() will overwrite values from the first array with the second if keys are shared:

$first = ['item1' => 'value1', 'item2' => 'value2'];
$second = ['item1' => 'value3', 'item3' => 'value4'];

$merged = array_merge($first, $second);

var_dump($merged);

Output:

array(3) {
  ["item1"]=> string(6) "value3"
  ["item2"]=> string(6) "value2"
  ["item3"]=> string(6) "value4"
}

In this case, the value of item1 in the first array was replaced by the value in the second array.

Key Considerations When Choosing Between + and array_merge()

1. Preserving Original Data: If you need to maintain the original values from the first array when overlapping keys exist, use the + operator. If you want to ensure that values from the second array overwrite the first, use array_merge().
2. Handling Non-Associative Arrays: For non-associative arrays (with numerical indexes), the + operator will not reorder the keys. In contrast, array_merge() will reindex the array starting from 0, which could be problematic if the original order is significant.
3. Performance: For large arrays, consider the performance implications. While both methods perform well for most use cases, array_merge() may have slightly more overhead due to its key comparison and value overwriting.

Edge Cases to Consider

• Merging Empty Arrays: If one of the arrays is empty, the other array will be returned as is.
• Multidimensional Arrays: The + operator merges at the first level only. If you need to merge arrays recursively (e.g., nested arrays), consider a custom solution or using array_merge_recursive().

$first = ['item1' => ['subitem1' => 'value1']];
$second = ['item1' => ['subitem2' => 'value2'], 'item2' => 'value3'];

$merged = array_merge_recursive($first, $second);

var_dump($merged);

Output:

array(2) {
  ["item1"]=> array(2) {
    ["subitem1"]=> string(6) "value1"
    ["subitem2"]=> string(6) "value2"
  }
  ["item2"]=> string(6) "value3"
}

Finally

The + operator is a simple and effective way to merge arrays in PHP when you want to avoid key value overwriting and maintain the original values of the first array. Understanding the difference between the + operator and array_merge() will help you choose the right tool for your specific use case. Keep in mind the edge cases, and make sure you pick the merging strategy that aligns with your data handling needs.

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