How to Use array_unique in PHP: Remove Duplicate Values Easily

Use the array_unique function in PHP to remove duplicate values from an array. It returns a new array with only unique values, preserving the original keys.

Syntax

The array_unique function takes an array as input and returns a new array with duplicate values removed. It has an optional second parameter to specify how duplicates are compared.

• array_unique(array $array, int $flags = SORT_STRING): array
• $array: The input array to filter.
• $flags: Optional. Defines the comparison type (e.g., SORT_STRING, SORT_NUMERIC, SORT_REGULAR).

php

array_unique(array $array, int $flags = SORT_STRING): array

Example

This example shows how to use array_unique to remove duplicate values from an array of fruits. The function returns a new array with only unique fruits, preserving the original keys.

php

<?php
$fruits = ['apple', 'banana', 'apple', 'orange', 'banana', 'grape'];
$uniqueFruits = array_unique($fruits);
print_r($uniqueFruits);
?>

Output

Array
(
    [0] => apple
    [1] => banana
    [3] => orange
    [5] => grape
)

Common Pitfalls

One common mistake is expecting array_unique to reindex the array keys. It preserves original keys, which can cause unexpected results if you rely on sequential keys. To fix this, use array_values to reindex the array after removing duplicates.

Another pitfall is misunderstanding the $flags parameter, which affects how values are compared (string vs numeric).

php

<?php
// Wrong: expecting keys to be reindexed
$numbers = [1, 2, 2, 3];
$uniqueNumbers = array_unique($numbers);
print_r($uniqueNumbers); // keys are 0,1,3

// Right: reindex keys after unique
$uniqueNumbersReindexed = array_values(array_unique($numbers));
print_r($uniqueNumbersReindexed); // keys are 0,1,2
?>

Output

Array
(
    [0] => 1
    [1] => 2
    [3] => 3
)
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
)

Quick Reference

Parameter	Description
$array	The input array to remove duplicates from.
$flags	Optional. Comparison type: SORT_STRING (default), SORT_NUMERIC, SORT_REGULAR.
Return	A new array with duplicate values removed, original keys preserved.

Key Takeaways

Use array_unique to remove duplicate values from an array while preserving keys.

Remember array_unique does not reindex keys; use array_values to reset keys if needed.

The optional flags parameter controls how values are compared for uniqueness.

array_unique works on values only, not keys.

Always test output to understand how keys and values are handled.