Mastering PHP’s array_flip: Reverse Keys and Values with Confidence

In PHP, the array_flip() function is a frequently used utility that swaps the keys and values of an array, returning a new array with the reversed mapping.

Basic Syntax

array array_flip ( array $array )

Parameter Description

The sole parameter $array is the array you want to flip.

Return Value

The function returns the flipped array. If the supplied argument is not a valid array, it returns bool(false).

Example 1: Simple Flip

$array = array(
    'a' => 'apple',
    'b' => 'banana',
    'c' => 'cherry'
);
$flippedArray = array_flip($array);
print_r($flippedArray);

Output:

Array
(
    [apple] => a
    [banana] => b
    [cherry] => c
)

This example creates an associative array with three key‑value pairs, flips it, and prints the resulting array where the original values become keys.

Important Note on Duplicate Values

If the original array contains duplicate values, only the last occurrence is retained after flipping because keys must be unique. The earlier duplicates are overwritten.

Example 2: Duplicate Value Handling

$array = array(
    'a' => 'apple',
    'b' => 'banana',
    'c' => 'banana'
);
$flippedArray = array_flip($array);
print_r($flippedArray);

Output:

Array
(
    [apple] => a
    [banana] => c
)

Here, the value banana appears twice. After flipping, only the last key ( 'c') is kept, demonstrating that duplicate values lead to data loss in the flipped array.

Final Caution

When using array_flip(), ensure that the original array’s values are unique or can serve as unique string keys; otherwise, the result may not match expectations.