In PHP, why do we use the array_flip() function on an associative array?

Introduction / Context:
In PHP, arrays are one of the most important data structures, and associative arrays in particular are widely used to map keys to values. The array_flip() function is frequently asked about in interviews because it looks simple but solves a very specific problem: quickly reversing the relationship between keys and values. Understanding when and why developers use array_flip() helps clarify how associative arrays can be used as lookup tables and how to design cleaner code when working with mappings and reference data.

Given Data / Assumptions:

• We are working with PHP arrays, especially associative arrays where keys are usually strings and values may be strings or numbers.
• array_flip() takes one array argument and returns a new array.
• Values in the original array must be valid array keys in PHP (for example, integers or strings).
• If duplicate values exist in the original array, later keys will overwrite earlier ones in the flipped result.

Concept / Approach:
The main concept behind array_flip() is to turn values into keys so that lookups can be done by what was originally a value. This is very useful when the values are unique identifiers or codes and you want a fast way to check membership or map back from a value to its original key. Because array keys in PHP are optimised for direct access, using reversed mappings often provides better performance and cleaner code than searching linearly through values with loops or in_array().

Step-by-Step Solution:
Step 1: Start with an associative array such as $roles = array("admin" => 1, "editor" => 2, "viewer" => 3). Step 2: If you need to know which role name corresponds to an id, you could loop over the array to find which entry has value 2. Step 3: Instead, you can call array_flip($roles), which produces a new array like array(1 => "admin", 2 => "editor", 3 => "viewer"). Step 4: Now you can do a direct lookup $flipped[2] to get "editor" without searching. Step 5: This demonstrates that the primary purpose of array_flip() is to swap keys and values in order to support reverse lookups and alternative indexing.

Verification / Alternative check:
You can quickly verify the behaviour of array_flip() by printing the result of a simple example script that builds an associative array and then calls array_flip(). The output will show values turned into keys and keys turned into values. Testing with duplicate values will show that only the last one is kept in the flipped array, confirming that the function is intended for situations where values are unique or where overwriting is acceptable.

Why Other Options Are Wrong:
Option b is wrong because sorting is done with functions such as sort(), asort(), or ksort(), not with array_flip(). Option c is incorrect because array_flip() does not explicitly remove duplicates; collisions may overwrite earlier entries but that is a side effect, not the primary purpose. Option d is wrong because merging arrays is done with array_merge() or the plus operator for arrays, not with array_flip().

Common Pitfalls:
A common pitfall is to use array_flip() on arrays where values are not unique, leading to unexpected data loss when keys are overwritten. Another is to apply it on arrays where values are not valid keys, such as arrays or objects, which can cause warnings. Developers should therefore make sure that the values are suitable and preferably unique before flipping. When used correctly, array_flip() is a convenient tool for reverse lookups and simple mapping transformations.

Final Answer:
We use array_flip() in PHP to swap the keys and values of an array so that values become keys and keys become values, which is useful for reverse lookups and alternative indexing.