Interview question: How to use array_flip() to determine whether the array value is unique?

M66 2025-06-03

In PHP, array_flip() is a very practical array function that can syndicate keys and values in an array. This feature can be cleverly used to judge an array.

Function introduction

 array array_flip(array $array)

This function will return a new array, turning the key in the original array into a value , and the value into a key . But it should be noted that if there are duplicate values in the original array, array_flip() will overwrite the previous key and only the last one is retained.

Therefore, we can judge whether there are duplicate values by comparing the lengths of the original array and the flipped array. If the lengths of the two are inconsistent, it means that the values in the original array are not unique.

Sample code

 function isArrayValuesUnique(array $arr): bool {
    $flipped = array_flip($arr);
    return count($arr) === count($flipped);
}

// Test cases
$uniqueArray = ['a' => 'apple', 'b' => 'banana', 'c' => 'cherry'];
$nonUniqueArray = ['a' => 'apple', 'b' => 'banana', 'c' => 'apple'];

echo "Unique array test results：";
echo isArrayValuesUnique($uniqueArray) ? 'It&#39;s the only one' : 'There are duplications';
echo "\n";

echo "非Unique array test results：";
echo isArrayValuesUnique($nonUniqueArray) ? 'It&#39;s the only one' : 'There are duplications';

Output:

 Unique array test results：It&#39;s the only one
非Unique array test results：There are duplications

Examples of application scenarios

Suppose you have a user registration system and need to determine whether multiple fields entered by the user (such as email, username) have duplicate values. You can use this method to quickly judge:

 $userInputs = [
    'email1' => 'user1@m66.net',
    'email2' => 'user2@m66.net',
    'email3' => 'user1@m66.net', // repeat
];

if (!isArrayValuesUnique($userInputs)) {
    echo "存在repeat的用户信息，Check, please！";
} else {
    echo "All user information is unique，Continue to process。";
}

Summarize

Using array_flip() to determine whether the array value is unique is a concise and efficient technique. Its core principle lies in the uniqueness of array keys in PHP. Once repeated values occur, they will be automatically deduplicated during the flip process. We only need to compare the lengths.

This method is suitable for judging the value uniqueness of a one-dimensional array, and is a tip that is worth mastering in interviews or actual development.

array_flip