相对类类型
这些类型声明只能在类中使用。
self
The value must be an instanceof the same class as the one
in which the type declaration is used.
parent
值必须是 instanceof 使用了类型声明的父级类。
static
static is a return-only type which requires that the
value returned must be an instanceof the same class as the one
the method is called in.
自 PHP 8.0.0 起可用。
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