Combining Arrays in PHP

Arrays are fundamental data structures in PHP, and often you’ll need to combine the contents of two or more arrays into a single array. This tutorial explores various methods for achieving this, highlighting the most efficient and appropriate techniques for different scenarios.

Understanding the Need for Array Combination

Combining arrays is a common task when processing data from multiple sources, building up lists dynamically, or preparing data for further operations. Choosing the right method is important for performance and maintaining data integrity.

Basic Array Combination with `array_merge()`

The array_merge() function is the primary and most straightforward way to combine arrays in PHP. It takes one or more arrays as arguments and returns a new array containing all the elements from the input arrays in the order they are provided.

<?php
$array1 = array('a', 'b');
$array2 = array('c', 'd');

$combinedArray = array_merge($array1, $array2);

print_r($combinedArray); // Output: Array ( [0] => a [1] => b [2] => c [3] => d )
?>

array_merge() creates a new array, leaving the original arrays unchanged. This is generally preferred to avoid unintended side effects.

Important Considerations with array_merge():

Numeric Keys: If the input arrays have numeric keys, array_merge() will re-index the resulting array, starting from 0. This can be useful in some cases, but might not be desirable if you need to preserve the original keys.
String Keys: If the input arrays have string keys, array_merge() will preserve them. If multiple arrays have the same string key, the value from the last array will overwrite any previous values for that key.

Combining Arrays with the Spread Operator (PHP 7.4+)

PHP 7.4 introduced the spread operator (...), providing a concise and elegant way to combine arrays.

<?php
$array1 = array('a', 'b');
$array2 = array('c', 'd');

$combinedArray = [...$array1, ...$array2];

print_r($combinedArray); // Output: Array ( [0] => a [1] => b [2] => c [3] => d )
?>

The spread operator unpacks the elements of each array into a new array. It’s particularly useful when creating new arrays with a mix of static values and array elements.

Caution: The spread operator requires PHP 7.4 or later.

Using the `+` Operator for Array Combination

The + operator can also be used to combine arrays, but it behaves differently than array_merge(). It preserves the keys of the first array and adds any keys from subsequent arrays only if they don’t already exist in the first array.

<?php
$array1 = array('a' => 1, 'b' => 2);
$array2 = array('b' => 3, 'c' => 4);

$combinedArray = $array1 + $array2;

print_r($combinedArray); // Output: Array ( [a] => 1 [b] => 2 [c] => 4 )
?>

Notice that the value of ‘b’ in $array1 takes precedence and the value from $array2 is discarded. The + operator is useful when you want to merge arrays while giving priority to the keys of the first array.

Combining Arrays with Numeric Keys and Resetting the Index

If you have arrays with numeric keys and want to combine them into a single array with a contiguous, zero-based index, you can use array_merge() in combination with array_values().

<?php
$array1 = array(5 => 'a', 6 => 'b');
$array2 = array(8 => 'c', 9 => 'd');

$combinedArray = array_merge(array_values($array1), array_values($array2));

print_r($combinedArray); // Output: Array ( [0] => a [1] => b [2] => c [3] => d )
?>

array_values() extracts all the values from an array, re-indexing them from 0. Then, array_merge() combines the re-indexed arrays.

Choosing the Right Method

The best method for combining arrays depends on your specific needs:

array_merge(): The most versatile and commonly used method. Suitable for most scenarios, especially when you want to combine arrays with string keys or re-index arrays with numeric keys.
Spread Operator (...): A concise and elegant option for PHP 7.4+, particularly when creating new arrays with a mix of static values and array elements.
+ Operator: Useful when you want to prioritize the keys of the first array and avoid overwriting existing keys.
array_merge() with array_values(): Suitable when you want to combine arrays with numeric keys and reset the index to start from 0.

By understanding these different techniques, you can choose the most efficient and appropriate method for combining arrays in your PHP applications.