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

PHP provides a powerful function array_flip() that exchanges the keys and values of an array. This article introduces the function, its syntax, and practical usage through clear code examples.

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 while earlier duplicates are discarded.

Example 1 demonstrates flipping a fruit array:

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

$flipped_fruits = array_flip($fruits);

print_r($flipped_fruits);
?>

The output is:

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

This shows that the original keys "apple", "orange", and "banana" have become the values "red", "orange", and "yellow" in the flipped array.

In real‑world code, array_flip() can be used to look up a key by its value. After flipping, you can use isset() to check for the existence of a specific value.

Example 2 flips a student‑age array to find a student by age:

<?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";
}
?>

The script outputs:

The student with age 20 is John

This example shows how flipping the array allows direct retrieval of the student name from an age value.

Overall, mastering array_flip() enhances PHP array manipulation, enabling efficient key‑value swaps, duplicate handling, and lookup operations, thereby improving code readability and performance.