How to remove duplicate values from array using PHP?

In this article, we will discuss removing duplicate elements from an array in PHP. We can get the unique elements by using array_unique() function. This function will remove the duplicate values from the array.

Syntax:

array array_unique($array, $sort_flags);

Note: The keys of the array are preserved i.e. the keys of the not removed elements of the input array will be the same in the output array.

Parameters:

This function accepts two parameters that are discussed below:

• $array: This parameter is mandatory to be supplied and it specifies the input array from which we want to remove duplicates.
• $sort_flags: It is an optional parameter. This parameter may be used to modify the sorting behavior using these values:
  • SORT_REGULAR: This is the default value of the parameter $sort_flags. This value tells the function to compare items normally (don’t change types).
  • SORT_NUMERIC: This value tells the function to compare items numerically.
  • SORT_STRING: This value tells the function to compare items as strings.
  • SORT_LOCALE_STRING: This value tells the function to compare items as strings, based on the current locale.

Return Value:

The array_unique() function returns the filtered array after removing all duplicates from the array.

Using array_unique() function

The array_unique() function in PHP removes duplicate values from an array by comparing the values and removing subsequent occurrences. It returns a new array with only unique values while preserving the original order of elements. This function is simple and efficient for deduplicating arrays in PHP applications.

Example 1: PHP program to remove duplicate values from the array.

<?php
  
// Input Array
$a = array(
    "red",
    "green",
    "red",
    "blue"
);

// Array after removing duplicates
print_r(array_unique($a));
?>

Output:

Array
(
    [0] => red
    [1] => green
    [3] => blue
)

Example 2: PHP program to remove duplicate elements from an associative array.

<?php
  
// Input array
$arr = array(
    "a" => "MH",
    "b" => "JK",
    "c" => "JK",
    "d" => "OR"
);

// Array after removing duplicates
print_r(array_unique($arr));
?>

Output:

Array
(
    [a] => MH 
    [b] => JK 
    [d] => OR 
)

Example 3: This is another way of removing the duplicates from the array using the PHP in_array() method.

<?php
  
// This is the array in which values
// will be stored after removing
// the duplicates
$finalArray = array();

// Input array
$arr = array(
    "a" => "MH",
    "b" => "JK",
    "c" => "JK",
    "d" => "OR"
);
foreach ($arr as $key => $value)
{
    // If the value is already not stored
    // in the final array
    if (!in_array($value, $finalArray)) $finalArray[$key] = $value;
} 
// End foreach
print_r($finalArray);

?>

Output:

Array
(
  [a] => MH 
  [b] => JK 
  [d] => OR 
)

Using array_filter()

To remove duplicate values from an array using `array_filter()`, apply a callback function that checks each element's index against its first occurrence. Return true only for the first occurrence to filter out duplicates.

Example: This example shows the use of the above-mentioned approach.

<?php
$array = [1, 2, 2, 3, 4, 4, 5];

$uniqueArray = array_filter($array, function($value, $key) use ($array) {
    return array_search($value, $array) === $key;
}, ARRAY_FILTER_USE_BOTH);

print_r(array_values($uniqueArray));
?>

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

Using Iteration and Comparison

To remove duplicate values from an array in PHP without using `array_unique()`, iterate through the array, check each value's existence in a separate array (`$uniqueArray`), and append it if not already present.

Example: This example shows the use of the above-mentioned approach.

<?php
function removeDuplicates($array) {
    $uniqueArray = [];
    
    foreach ($array as $value) {
        // Check if the value is already in $uniqueArray
        if (!in_array($value, $uniqueArray)) {
            $uniqueArray[] = $value;
        }
    }
    
    return $uniqueArray;
}

// Example array with duplicate values
$array = [1, 2, 2, 3, 4, 4, 5];

// Remove duplicates
$uniqueArray = removeDuplicates($array);

// Print the unique array
print_r($uniqueArray);
?>

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

Using array_flip() and array_values() Functions:

The array_flip() function in PHP exchanges keys with their associated values in an array, effectively removing duplicate values. `array_values()` is then used to reset the array indices, providing a cleaned-up array without duplicate values while maintaining the original order.

Example

<?php
$array = array("apple", "banana", "cherry", "apple", "banana");

$uniqueArray = array_values(array_flip($array));

print_r($uniqueArray);
?>

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

Using array_count_values()

The `array_count_values()` function counts all the values of an array and returns an associative array where keys are the original values and values are their counts. To remove duplicates, filter keys with a count of 1, and then use `array_keys()` to extract unique values.

Example:

<?php
$array = [1, 2, 2, 3, 4, 4, 5];

// Count values
$valueCounts = array_count_values($array);

// Filter to keep only values that appear once
$uniqueValues = array_keys(array_filter($valueCounts, function($count) {
    return $count === 1;
}));

print_r($uniqueValues);
?>

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