How to Use array_merge in PHP: Syntax and Examples

Use array_merge in PHP to combine two or more arrays into one. It appends elements from later arrays to the first array, reindexing numeric keys and preserving string keys.

Syntax

The array_merge function takes one or more arrays as arguments and returns a new array that combines all elements.

• array_merge(array1, array2, ...): Combines arrays in the order given.
• Numeric keys are reindexed starting from 0.
• String keys are preserved; if keys repeat, later values overwrite earlier ones.

php

array array_merge(array ...$arrays)

Example

This example shows how array_merge combines two arrays with numeric and string keys.

php

<?php
$array1 = ["apple", "banana"];
$array2 = ["cherry", "date"];
$result = array_merge($array1, $array2);
print_r($result);

// Example with string keys
$array3 = ["a" => "red", "b" => "green"];
$array4 = ["b" => "blue", "c" => "yellow"];
$result2 = array_merge($array3, $array4);
print_r($result2);
?>

Output

Array
(
    [0] => apple
    [1] => banana
    [2] => cherry
    [3] => date
)
Array
(
    [a] => red
    [b] => blue
    [c] => yellow
)

Common Pitfalls

Common mistakes when using array_merge include:

• Expecting numeric keys to be preserved (they are reindexed).
• Not realizing string keys with the same name get overwritten by later arrays.
• Using + (array union) operator instead, which behaves differently.

Here is a wrong and right way example:

php

<?php
// Wrong: expecting numeric keys to stay the same
$array1 = [1 => "one", 2 => "two"];
$array2 = [3 => "three"];
$result = array_merge($array1, $array2);
print_r($result); // keys become 0,1,2

// Right: understand keys are reindexed
// Use array_merge when you want a combined list ignoring original keys
?>

Output

Array
(
    [0] => one
    [1] => two
    [2] => three
)

Quick Reference

Tips for using array_merge:

• Use it to combine arrays into one list.
• Numeric keys are reindexed starting at 0.
• String keys are preserved; later values overwrite earlier ones.
• For preserving keys without overwriting, consider array_replace or the + operator.

Key Takeaways

array_merge combines multiple arrays into one, reindexing numeric keys.

String keys are preserved and overwritten by later arrays if duplicated.

Use array_merge when you want a merged list ignoring original numeric keys.

Be careful: numeric keys do not stay the same after merging.

For preserving keys without overwriting, use alternatives like array_replace or + operator.