How to solve the problem of using object type parameters in PHP interface that cause type incompatibility?

PHP interface and type compatibility: Avoid the trap of object type parameters

In PHP development, the mismatch between the interface definition and the implementation class type often leads to the "must be compatible with" error. This article analyzes such problems that arise in a PHP interface definition and implementation class and provides solutions.

Problem: The interface is incompatible with the implementation class type

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

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

The parameter type of save method in the class is AdminRequest :

 class AdminController implements IAdminController
{
    public function save(AdminRequest $request): array
    {
        // ...Method body...
    }
}

Although AdminRequest is a subclass of object (all classes are implicitly inherited from object ), the runtime still reports an error "must be compatible with". Even if gettype($adminRequest) returns object , it still cannot pass the type check.

Cause analysis and solutions

Using object as method parameter type is not best practice. Although all classes inherit from object , this definition is too broad and lacks type safety guarantees. When checking the type, the PHP interpreter will strictly verify whether the specific type of the parameter exactly matches the interface definition. object as a parameter type does not constrain the specific type, and even if AdminRequest is an object, it does not conform to the object type specified in the interface definition.

A better solution is to use more specific types as interface parameters. If AdminRequest , UserRequest and other classes inherit from Laravel's Request class, the parameter type of the save method is defined as \Illuminate\Http\Request in the interface:

 interface IAdminController
{
    // ...Other methods...
    public function save(\Illuminate\Http\Request $request): array;
    // ...Other methods...
}

In this way, all classes inherited from \Illuminate\Http\Request can be used as parameters of the save method, solving the problem of type incompatibility. This takes advantage of the covariant nature of the PHP type system, allowing subclass parameter signatures to be looser than parent classes.

Important tips:

• In versions prior to PHP 7.2, object has more restrictions as method signature type. Although PHP 7.2 and above support object as parameter type, it is still not recommended to use in interface definitions.
• Using more specific types can improve code readability, maintainability, and type safety.
• If you have to use object for type checking, you can use is_a() function for explicit type checking inside the method, but this will reduce the type safety of the code and reduce readability. Try to avoid this practice.

By selecting a more specific type definition interface, you can effectively avoid type incompatibility errors and improve code quality.

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