Classes should be open for extension but closed for modification.

You should be able to add a new payment method (e.g., Stripe) by creating a new class that extends a base interface, rather than rewriting your existing checkout logic. L: Liskov Substitution Principle (LSP)

If a Bird class has a fly() method, a Penguin subclass shouldn't override it to throw an error, as this violates the expectation that all birds in the system can fly. I: Interface Segregation Principle (ISP)

Php-guide.7z

Classes should be open for extension but closed for modification.

You should be able to add a new payment method (e.g., Stripe) by creating a new class that extends a base interface, rather than rewriting your existing checkout logic. L: Liskov Substitution Principle (LSP)

If a Bird class has a fly() method, a Penguin subclass shouldn't override it to throw an error, as this violates the expectation that all birds in the system can fly. I: Interface Segregation Principle (ISP)