The term SOLID is a mnemonic acronym for five design principles intended to make software designs more understandable, flexible and maintainable. The principles are a subset of many principles promoted by Robert C. Martin. Though they apply to any object-oriented design, the SOLID principles can also form a core philosophy for methodologies such as agile development or Adaptive Software Development. The SOLID acronym was introduced by Michael Feathers.
Basically these principles are for designing/defining a class, inheritance and architecture in Object oriented programming.
There are 5 post regarding all 5 principles in which I have explained principles one by one with appropriate examples in PHP.