Single responsibility principle
A class should have only a single responsibility
Software entities should be open for extension, but closed for modification.
Liskov substitution principle
Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.
Interface segregation principle
Many client-specific interfaces are better than one general-purpose interface.
Dependency inversion principle
One should depend upon abstractions, [not] concretions.
PHP code: https://github.com/mihaimitrut/solid