Are there any built-in PHP functions that can be used as the opposite of "isset"?

When checking if a variable is set in PHP, the `isset` function is commonly used. However, if you want to check if a variable is not set, there isn't a built-in function specifically for that purpose. One way to achieve the opposite of `isset` is to use the `!` (not) operator in combination with `isset`. By negating the result of `isset`, you can effectively check if a variable is not set.

// Check if a variable is not set
if (!isset($variable)) {
    echo "Variable is not set";
}