How to Merge Two Arrays in PHP: Simple Guide

In PHP, you can merge two arrays using the array_merge() function which combines elements from both arrays. Alternatively, you can use the + operator to merge arrays while preserving keys from the first array.

Syntax

The main ways to merge arrays in PHP are:

• array_merge(array1, array2, ...): Combines arrays by appending elements and reindexing numeric keys.
• array1 + array2: Adds elements from array2 to array1 only if keys do not exist in array1.

php

<?php
// Using array_merge
$result = array_merge($array1, $array2);

// Using + operator
$result = $array1 + $array2;
?>

Example

This example shows how array_merge() and the + operator merge two arrays differently.

php

<?php
$array1 = ["a" => "apple", "b" => "banana"];
$array2 = ["b" => "blueberry", "c" => "cherry"];

// Merge with array_merge
$merged = array_merge($array1, $array2);
print_r($merged);

// Merge with + operator
$union = $array1 + $array2;
print_r($union);
?>

Output

Array
(
    [a] => apple
    [b] => blueberry
    [c] => cherry
)
Array
(
    [a] => apple
    [b] => banana
    [c] => cherry
)

Common Pitfalls

Common mistakes when merging arrays include:

• Expecting array_merge() to preserve keys for numeric arrays; it reindexes numeric keys.
• Using the + operator and expecting it to overwrite keys; it does not overwrite existing keys in the first array.
• Not understanding that array_merge() appends values, which can duplicate keys if numeric.

php

<?php
// Wrong: expecting + to overwrite keys
$array1 = [1 => 'one', 2 => 'two'];
$array2 = [2 => 'deux', 3 => 'trois'];
$result = $array1 + $array2;
print_r($result); // key 2 stays 'two', not 'deux'

// Right: use array_merge to overwrite
$result = array_merge($array1, $array2);
print_r($result); // keys reindexed, values appended
?>

Output

Array
(
    [1] => one
    [2] => two
    [3] => trois
)
Array
(
    [0] => one
    [1] => two
    [2] => deux
    [3] => trois
)

Quick Reference

Summary of array merging methods:

Method	Behavior	Key Handling
array_merge()	Appends arrays, reindexes numeric keys	Numeric keys reindexed, string keys overwritten
+ operator	Adds elements from second array only if key not in first	Preserves keys, no overwriting

Key Takeaways

Use array_merge() to combine arrays and overwrite string keys, but numeric keys get reindexed.

Use the + operator to merge arrays without overwriting existing keys in the first array.

Understand how keys are handled to avoid unexpected results when merging arrays.

Test your merge method with your specific arrays to ensure it behaves as you expect.