What does the `isset()` function do in PHP?

The isset() function is designed to determine if a variable has been declared and is not NULL. When you call isset() with a variable as its argument, the function returns TRUE if the variable exists and is not NULL; otherwise, it returns FALSE. This function is particularly useful for checking if variables are available for use, especially when working with form data, session variables, or any situation where you might not be sure if a variable has been initialized or assigned a value.

In contrast, the other options describe different behaviors that isset() does not perform. Fetching the value of a variable would involve simply echoing or returning the variable itself, which is not what isset() is designed to do. Creating a variable if it is not set is also not a function of isset(), as it only checks for existence rather than modifying the variable's state. Lastly, deleting a variable from memory is handled by the unset() function in PHP, which specifically removes a variable, while isset() does not modify the variable's state at all.