Understanding PHP's array_unique() Function: Syntax, Parameters, Examples, and Best Practices

PHP is a widely used server‑side scripting language that offers many built‑in functions for handling mathematics, strings, arrays, files, etc. Among them, the array_unique() function is essential for removing duplicate values from an array.

Function purpose : array_unique() returns a new array containing only the first occurrence of each value, effectively eliminating duplicates.

Syntax :

<code>array_unique(array $array, int $sort_flags = SORT_STRING): array</code>

Parameters:

$array (required) – the input array to be processed.

$sort_flags (optional) – defines how values are compared. Options include SORT_REGULAR , SORT_NUMERIC , SORT_STRING , SORT_LOCALE_STRING , SORT_NATURAL , and the case‑insensitive flag SORT_FLAG_CASE which can be combined with SORT_STRING or SORT_NATURAL .

The function returns the processed array.

Example :

<code>&lt;?php
$array = array(1, 2, 3, 2, 4);
$result = array_unique($array);
print_r($result);
?&gt;
</code>

Output:

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

Notes : When the input array contains duplicate keys, array_unique() keeps the first key/value pair and discards the rest. For large arrays the function can be performance‑intensive, so developers should consider the size of the data set and possible alternatives.

Conclusion : array_unique() is a convenient PHP array function for deduplication, but its parameters, return format, and performance impact should be understood to ensure stable and efficient code.