Keep interfaces small

How to do it

Shape each interface around one caller's needs. A class may implement several small interfaces. If an implementation leaves methods empty or throwing, the interface is too big.

Example

// Bad: a printer must pretend to scan and fax
interface Machine { public function print(Doc $d): void; public function scan(): Doc; public function fax(Doc $d): void; }

// Good: one interface per role
interface Printer { public function print(Doc $d): void; }
interface Scanner { public function scan(): Doc; }

// Good: one interface per role
interface Printer { print(doc: Doc): void }
interface Scanner { scan(): Doc }
class OfficePrinter implements Printer { print(doc: Doc) { /* ... */ } }

How to check

In review, no implementation has empty or throwing methods. A linter such as PHPMD TooManyMethods flags fat interfaces.