How can changes in abstract classes affect the reusability of singleton pattern in PHP?

Changes in abstract classes can affect the reusability of the singleton pattern in PHP because abstract classes cannot be instantiated directly. If the singleton pattern relies on a specific abstract class, any changes to that abstract class may break the singleton pattern implementation. To solve this issue, you can create a separate concrete class that extends the abstract class and use this concrete class in the singleton pattern implementation.

abstract class AbstractClass {
    // Abstract class implementation
}

class ConcreteClass extends AbstractClass {
    // Concrete class implementation
}

class Singleton {
    private static $instance;

    public static function getInstance() {
        if (self::$instance === null) {
            self::$instance = new ConcreteClass();
        }
        return self::$instance;
    }
}