What potential pitfalls should PHP developers be aware of when using isset with Closure objects?

When using isset with Closure objects in PHP, developers should be aware that isset will always return false for Closure objects, regardless of whether the Closure is defined or not. To check if a Closure object is set or not, developers should use the is_callable function instead. This function will correctly determine if a given variable is a Closure object or not.

$closure = function() {
    echo "Hello, world!";
};

if (is_callable($closure)) {
    $closure();
} else {
    echo "Closure is not set.";
}