手册里，没有规定抽象类就一定要包含抽象方法，有些手册翻译有误，详见官方文档。
PHP 5 introduces abstract classes and methods. It is not allowed to create an instance of a class that has been defined as abstract. Any class that contains at least one abstract method must also be abstract. Methods defined as abstract simply declare the method's signature they cannot define the implementation.
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