Master PHP’s array_flip(): Swap Keys and Values with Real Code Examples

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

Syntax

array array_flip ( array $array )

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

Basic Example

<?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", "banana") become the values, and the original values become the keys.

Practical Use Case: Finding a Key by Value

When you need to locate a key based on a value, you can first flip the array and then use isset() to check for the desired 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";
}
?>

The script outputs: The student with age 20 is John In this example, the associative array $students is flipped so that ages become keys, allowing a quick lookup of the corresponding student name.

Conclusion

The array_flip() function is a powerful tool in PHP for exchanging array keys and values, simplifying tasks such as reverse lookups and removing duplicate values, thereby improving code readability and efficiency.