Understanding PHP's array_flip() Function: Usage, Syntax, and Examples

In PHP programming, arrays are a common data structure, and the array_flip() function is a useful built‑in function that swaps array keys and values.

The function takes an array as its sole argument and returns a new array where the original keys become values and the original values become keys. The values must be strings or integers; otherwise an error occurs.

Syntax:

array array_flip ( array $array )

Parameter:

$array : the array whose keys and values are to be exchanged.

Return value:

The function returns the flipped array. If any value in the input array is not a string or integer, a warning is raised.

Example 1:

$array = array("a" => 1, "b" => 2, "c" => 3);
$flippedArray = array_flip($array);
print_r($flippedArray);

Output:

Array
(
    [1] => a
    [2] => b
    [3] => c
)

Example 2 demonstrates flipping an associative array where keys are fruit names and values are colors.

$array = array("apple" => "red", "banana" => "yellow", "orange" => "orange", "grape" => "purple");
$flippedArray = array_flip($array);
print_r($flippedArray);

Output:

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

Example 3 shows flipping a numerically indexed array with duplicate values; the resulting array keeps the last index for each value.

$array = array(1 => "a", 2 => "b", 3 => "c", 4 => "a");
$flippedArray = array_flip($array);
print_r($flippedArray);

Output:

Array
(
    [a] => 4
    [b] => 2
    [c] => 3
)

In summary, array_flip() is a practical PHP function for swapping keys and values of an array, useful especially with associative arrays, provided that all values are strings or integers.