SOLID Principles coded in PHP scripting language.

1. Single responsibility

• a class or a module should have only a single responsibility
• there should be only one reason to change a class or module

2. Open/closed

• classes, functions, modules should be open for extension, but closed for modification

3. Liskov substitution

• if a program module is using a Base class then the reference to the Base class be replaced with a Derived class without affecting the functionality of the module.
  • subclass does not allow or ignores some operation that the base class provides
  • subclass does not allow some parameters or ignores them.

4. Interface segregation

• classes that implement interfaces should not be forced to implement methods they do not use
• it is better to have many small interfaces, rather than a few large interfaces

5. Dependency inversion

• high level objects should not depend on low level implementations. Both should depend on abstractions.
• abstractions should not depend on details. Details should depend on abstractions.