a) The isset() function is used to check whether a variable is set or not
Description: The isset() function is a built-in function of PHP, which is used to determine that a variable is set or not. If a variable is considered set, it means the variable is declared and has a different value from the NULL. In short, it checks that the variable is declared and not null. This function returns true if the variable is not null, otherwise it returns false.
