This built-in function of PHP is used to exchange elements within an array, i.e., exchange all keys with their associated values in an array and vice-versa. We must remember that the values of the array need to be valid keys, i.e. they need to be either integer or string. A warning will be thrown if a value has the wrong type, and the key/value pair in question will not be included in the result.
Examples:
Input : array = ("aakash" => 20, "rishav" => 40, "gaurav" => 60)
Output:
Array
(
[20] => aakash
[40] => rishav
[60] => gaurav
)
Explanation: The keys and values are exchanged and the last key or value is taken.
Input : array = ("aakash" => "rani", "rishav" => "sristi",
"gaurav" => "riya", "laxman" => "rani")
Output:
Array
(
[rani] => laxman
[sristi] => rishav
[riya] => gaurav
)
Syntax:
array array_flip($array)
Parameters: The function takes only one parameter $array that refers to the input array.
Return Type: This function returns another array, with the elements exchanged or flipped, it returns null if the input array is invalid.
Below program illustrates the working of array_flip() function in PHP:
Example 1:
<?php   // PHP function to illustrate the use of array_flip() function Flip($array) {     $result = array_flip($array);     return($result); }   $array = array("aakash" => "rani", "rishav" => "sristi",                "gaurav" => "riya", "laxman" => "rani"); print_r(Flip($array));   ?> |
Output:
Array
(
[rani] => laxman
[sristi] => rishav
[riya] => gaurav
)
If multiple values in the array are the same, then on the use of the array_flip() function, only the key with the maximum index (after swap) will be added to the array. (This is done to ensure that no keys have duplicates.)
Example 2:
<?php // PHP program to array_flip() with multiple similar values   //function to use array_flip() function Flip($array) {     $result = array_flip($array);     return($result); }     //make the array $array = array("a" => 1, "b" => 1, "c" => 2);   //print all resultant values print_r(Flip($array));   ?> |
Output:
Array
(
[1] => b
[2] => c
)
References: http://php.net/manual/en/function.array-flip.php
