PHP isset() Function: Purpose & Usage

In PHP, the isset function is used to check if a variable has been set and is not null. Its purpose includes:

1. Check if a variable has been initialized.
2. Check if the elements in an array have been set.
3. Check if an element in a superglobal variable ($_GET, $_POST, $_REQUEST, $_SESSION, $_COOKIE, $_SERVER, $_ENV, etc.) has been set.
4. Check if a property of an object has been set.

By using the isset function, you can avoid warnings or errors when accessing variables that have not been set. It is recommended to use the isset function before accessing variables to ensure they have been correctly set.