Object interfaces allow you to create code which specifies which methods a class must implement, without having to define how these methods are implemented. Interfaces share a namespace with classes and traits, so they may not use the same name.
Interfaces are defined in the same way as a class, but with the interface keyword replacing the class keyword and without any of the methods having their contents defined.
All methods declared in an interface must be public; this is the nature of an interface.
In practice, interfaces serve two complementary purposes:
- To allow developers to create objects of different classes that may be used interchangeably because they implement the same interface or interfaces. A common example is multiple database access services, multiple payment gateways, or different caching strategies. Different implementations may be swapped out without requiring any changes to the code that uses them.
- To allow a function or method to accept and operate on a parameter that conforms to an interface, while not caring what else the object may do or how it is implemented. These interfaces are often named like
Iterable,Cacheable,Renderable, or so on to describe the significance of the behavior.
Interfaces may define magic methods to require implementing classes to implement those methods.
Note:
Although they are supported, including constructors in interfaces is strongly discouraged. Doing so significantly reduces the flexibility of the object implementing the interface. Additionally, constructors are not enforced by inheritance rules, which can cause inconsistent and unexpected behavior.
implements
To implement an interface, the implements operator is used. All methods in the interface must be implemented within a class; failure to do so will result in a fatal error. Classes may implement more than one interface if desired by separating each interface with a comma.
Warning
A class can implement two interfaces which define a method with the same name, only if the method declaration in both interfaces is identical.
Warning
A class that implements an interface may use a different name for its parameters than the interface. However, as of PHP 8.0 the language supports named arguments, which means callers may rely on the parameter name in the interface. For that reason, it is strongly recommended that developers use the same parameter names as the interface being implemented.
Note:
Interfaces can be extended like classes using the extends operator.
Note:
The class implementing the interface must declare all methods in the interface with a compatible signature.
Constants
It's possible for interfaces to have constants. Interface constants work exactly like class constants. Prior to PHP 8.1.0, they cannot be overridden by a class/interface that inherits them.
Examples
Example #1 Interface example
Example #2 Extendable Interfaces
Example #3 Multiple interface inheritance
Example #4 Interfaces with constants
Example #5 Interfaces with abstract classes
Example #6 Extending and implementing simultaneously
An interface, together with type declarations, provides a good way to make sure that a particular object contains particular methods. See instanceof operator and type declarations.
thanhn2001 at gmail dot com ¶
11 years ago
PHP prevents interface a contant to be overridden by a class/interface that DIRECTLY inherits it. However, further inheritance allows it. That means that interface constants are not final as mentioned in a previous comment. Is this a bug or a feature?
vcnbianchi ¶
9 months ago
Just as all interface methods are public, all interface methods are abstract as well.
williebegoode at att dot net ¶
8 years ago
In their book on Design Patterns, Erich Gamma and his associates [AKA: "The Gang of Four"] use the term "interface" and "abstract class" interchangeably. In working with PHP and design patterns, the interface, while clearly a "contract" of what to include in an implementation is also a helpful guide for both re-use and making changes. As long as the implemented changes follow the interface [whether it is an interface or abstract class with abstract methods], large complex programs can be safely updated without having to re-code an entire program or module.
In PHP coding with object interfaces [as a keyword] and "interfaces" in the more general context of use that includes both object interfaces and abstract classes, the purpose of "loose binding" [loosely bound objects] for ease of change and re-use is a helpful way to think about both uses of the term "interface." The focus shifts from "contractual" to "loose binding" for the purpose of cooperative development and re-use.