Object type parameter compatibility problem in PHP interface: How to solve the 'must be compatible with' error?

In-depth discussion on the compatibility issues of PHP interface and object type

In PHP development, interfaces are an important tool for defining code structures and behaviors. However, in interface method parameter type declarations, type compatibility issues are often encountered, resulting in "must be compatible with" errors. This article will analyze this error in detail and provide effective solutions.

Problem scenario:

Suppose we define an interface IAdminController where the parameter type of the save method is declared as object :

 interface IAdminController {
    function save(object $request): array;
    // ... other methods
}

In the implementation class, the parameter type of the save method is AdminRequest :

 class AdminController implements IAdminController {
    function save(AdminRequest $request): array {
        // ... method implementation
    }
}

When running the code, you may get the " AdminRequest must be compatible with object " error, even if gettype($adminRequest) returns object .

Analysis of the root cause of the problem:

The key to the error is that the parameter type of the save method in the interface IAdminController is declared as object . Although AdminRequest is an instance of object type, PHP has very strict requirements on matching object types in type checking implemented by interfaces. In PHP 7.1 and previous versions, when object is used as a parameter type, the specific object type passed in must exactly match object . Although AdminRequest is an object, it is not object itself. Although PHP 7.2 and above are more relaxed in parameter hints for object type, this is not the core of the problem.

The root cause lies in the strictness of type matching. The interface defines a contract that the implementation class must be accurately abide by. As a parameter type, object is too broad to ensure the specific behavior and properties of the passed object.

Solution:

To solve this problem and avoid using too broad object types, we can adopt the following better practices:

1. Use more specific type tips: If all xxxRequest classes inherit from the same base class (such as Laravel's Request class), you can modify the parameter type of the interface save method to that base class.

2. Create a more specific interface: Create an interface (such as IRequest ) and let all xxxRequest classes implement the interface. Then, declare the save method parameter type of IAdminController interface as IRequest . This method not only ensures type safety, but also avoids the ambiguity of object types.

Best Practices:

It is not recommended to use object as the interface method parameter type directly because it limits the flexibility of the interface and easily leads to type compatibility issues. Selecting the appropriate parameter type according to actual needs, such as using the base class or custom interface provided by the framework, can effectively avoid such errors and improve the maintainability and readability of the code.

Through the above analysis and solutions, you can better understand the compatibility issues of object type parameters in the PHP interface and write more robust and easier to maintain PHP code.

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