Using PHP's array_flip() Function to Swap Keys and Values

PHP is a widely used server‑side scripting language that provides powerful functions for handling arrays and data. One particularly useful function is array_flip() , which swaps the keys and values of an array.

The basic syntax of array_flip() is:

array array_flip ( array $array )

The function accepts an array and returns a new array where the original keys become values and the original values become keys. If duplicate values exist, the last key is retained and earlier duplicates are discarded.

Below is a simple example that demonstrates the usage of array_flip() :

<?php
$fruits = array("apple" => "red", "orange" => "orange", "banana" => "yellow");

$flipped_fruits = array_flip($fruits);

print_r($flipped_fruits);
?>

The output of the above code is:

Array
(
    [red] => apple
    [orange] => orange
    [yellow] => banana
)

In this example, the associative array $fruits contains three elements. After applying array_flip() , the keys and values are exchanged and stored in $flipped_fruits , which is then printed with print_r() .

Another practical scenario is using array_flip() to look up a key by its value. By flipping the array first and then using isset() , you can check whether a specific value exists as a key in the flipped array.

<?php
$students = array("Tom" => 18, "John" => 20, "Mary" => 19);

$flipped_students = array_flip($students);

$age_to_find = 20;

if (isset($flipped_students[$age_to_find])) {
    $student_name = $flipped_students[$age_to_find];
    echo "The student with age $age_to_find is $student_name";
} else {
    echo "No student with age $age_to_find";
}
?>

This code outputs:

The student with age 20 is John

Here, the associative array $students maps student names to ages. After flipping, the ages become keys, allowing a quick lookup of the student name for a given age.

Overall, array_flip() is a powerful PHP function that simplifies operations such as key‑value look‑ups, removing duplicate values, and improving code readability and efficiency when handling array data.