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. One particularly useful function is array_flip() , which swaps the keys and values of an array.

The basic syntax of the function is:

array array_flip ( array $array )

It 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 function keeps the last key and discards earlier ones.

Example 1 demonstrates flipping a simple associative array of fruits:

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

$flipped_fruits = array_flip($fruits);

print_r($flipped_fruits);
?>

The output shows that the colors become keys and the fruit names become values:

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

A practical application is searching for a key by its value. By flipping the array first, you can use isset() to check for the existence of a value:

<?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

Through these examples, the article shows that array_flip() is a powerful tool for array manipulation in PHP, enabling tasks such as key‑value reversal, duplicate handling, and efficient look‑ups, thereby improving code readability and performance.