Using PHP’s array_unique() Function to Remove Duplicate Elements and Sort Arrays

In PHP development, removing duplicate elements from an array is a common task, and the built‑in array_unique() function provides a straightforward solution.

The function signature is:

array array_unique ( array $array [, int $sort_flags = SORT_STRING ] )

$array is the input array to be processed, and the optional $sort_flags parameter determines how the resulting array is sorted.

A basic example demonstrates deduplication:

<?php
// Define an array with duplicate values
$array = array(1, 2, 2, 3, 4, 4, 5);

// Remove duplicates
$result = array_unique($array);

// Output the result
print_r($result);
?>

The output is:

Array
(
    [0] => 1
    [1] => 2
    [3] => 3
    [4] => 4
    [6] => 5
)

Note that array_unique() keeps the first occurrence of each value and discards later duplicates, so the original order of unique elements is preserved.

The function works only with one‑dimensional arrays; handling multidimensional arrays requires custom logic such as recursion.

Beyond deduplication, array_unique() can also sort the array when a $sort_flags value is supplied. The available flags are:

SORT_REGULAR – regular comparison

SORT_NUMERIC – numeric comparison

SORT_STRING – string comparison

For example, sorting numerically while removing duplicates:

<?php
// Define an array with duplicate values
$array = array(4, 1, 3, 2, 2, 1, 4);

// Remove duplicates and sort numerically
$result = array_unique($array, SORT_NUMERIC);

// Output the result
print_r($result);
?>

The output shows the array sorted by numeric value:

Array
(
    [0] => 4
    [1] => 1
    [2] => 3
    [3] => 2
)

In summary, array_unique() is a useful PHP function for quickly eliminating duplicate entries and, with optional flags, sorting the resulting array, making it a versatile tool for backend developers.