PHP のメソッドのオーバーライドでパラメーターの型の互換性が強制されるのはなぜですか?

Understanding the Violation of Strict Standards in Parameter Overriding

In PHP, strict standards are enforced to ensure that the Liskov substitution principle (LSP) is not violated. LSP is an essential aspect of object-oriented programming that requires a subtype to be substitutable for its supertype without altering the program's behavior.

When overriding a method in a child class, PHP strictly checks whether the method signature matches that of the parent class. Consider the following example:

<code class="php">class Foo
{
    public function bar(array $bar) {}
}

class Baz extends Foo
{
    public function bar($bar) {}
}</code>

In this scenario, the child class Baz overrides the bar() method and changes the parameter type from array to a generic variable. This violates the strict standard because the overridden method's signature is incompatible with the parent method's signature.

To understand why this is a problem, let's recall the LSP. If we can substitute an object of type Baz for an object of type Foo, the program's behavior should remain unchanged. However, in this case, the parameter type mismatch would result in a potential runtime error or incorrect behavior.

By enforcing strict standard checking, PHP prevents the potential issues that could arise from violating the LSP. It ensures that overridden methods preserve the expected behavior and prevent unforeseen errors from occurring.

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