What is the difference between checking if a class is set and if it has been initialized in PHP?

Checking if a class is set in PHP means verifying if a class has been defined or declared, while checking if a class has been initialized refers to ensuring that an instance of the class has been created. To check if a class has been set, you can use the `class_exists()` function, and to check if a class has been initialized, you can use the `instanceof` operator.

// Check if a class is set
if (class_exists('ClassName')) {
    echo 'Class has been defined.';
} else {
    echo 'Class has not been defined.';
}

// Check if a class has been initialized
$obj = new ClassName();
if ($obj instanceof ClassName) {
    echo 'Class has been initialized.';
} else {
    echo 'Class has not been initialized.';
}