How is the time of obtaining a value by key in a data structure determined?

In data structures, the time to retrieve a value by key depends on the type of structure and the implementation method. For example, in associative arrays (hash tables), which are used in PHP for arrays with keys, the access time on average is O(1) — constant time, due to the quick computation of the key's hash.

However, in the worst case (with collisions), the time can increase to O(n). PHP arrays are implemented as hash tables, so key access is usually very fast.

Example:

$array = ['key' => 'value'];
echo $array['key']; // fast key access