如果返回值是混合的，使用什么类型的提示?
function foo ($a): mixed
{
if ($a == 1)
return true;
else
return 'foo';
}
var_dump(foo(1));
由于 mixed 不是真正的类型，因此会导致:
Since mixed is not a real type this results to:
致命错误:未捕获的类型错误:foo() 的返回值必须是混合实例...
Fatal error: Uncaught TypeError: Return value of foo() must be an instance of mixed...
有没有办法在 mixed 返回值中键入提示，或者在这种类型的场景中您根本不声明类型提示?
Is there a way to type hint a mixed return value or do you simply not declare type hints in this type of scenario?
推荐答案
类型提示用于对传递或返回的值实施一些限制.由于混合"将允许任何值，将其用作类型提示将毫无意义，因此请忽略它.
Type hints are there to enforce some restriction on the value passed or returned. Since "mixed" would allow any value at all, its use as a type hint would be pointless, so just omit it.
从 PHP 8.0 开始，您可以指定联合类型"，这将允许您将示例中的返回类型声明为 bool|string.(在联合中包含 false 也有一种特殊情况，如 false|string，因为它通常用于指示失败，但不适用于 true.)
From PHP 8.0 onwards, you can specify "union types", which would allow you to declare the return type in your example as bool|string. (There is also a special case for including false in the union, as in false|string, since it's commonly used to indicate failure, but not for true.)
作为 false|string 的替代方法，您可以使用 可空类型提示，从 PHP 7.1 开始提供., 如果你想在失败时返回 null 而不是 false，可以指定为 ?string.
As an alternative to false|string you can use nullable type hints, which are available from PHP 7.1 onwards. , and can be specified as ?string if you want to return null on a failure instead of false.
在旧版本中，您可以在文档块中记录函数的返回值.IDE 和文档生成器通常会理解语法 @return bool|string.即使您使用的是 PHP 8，您也可能希望使用它来添加为什么返回类型变化的描述，以及何时期望哪种类型.
In older versions, you can document the return value of your function, in a docblock. IDEs and documentation generators will generally understand the syntax @return bool|string. Even if you are using PHP 8, you might want to use this to add a description of why the return type varies, and when to expect which type.
当然，一个非常值得考虑的选择是重新考虑您的设计，并提出一个可以传达其结果的函数针对不同的情况返回不同的类型.例如，使用异常而不是错误的特殊值，或者将像 getThing(boolean $asString): float|string 这样的函数拆分为两个函数 getThingAsFloat(): float和 getThingAsString(): string.
Of course, an option well worth considering is to reconsider your design, and come up with a function which can communicate its result without returning different types for different cases. For instance, using exceptions instead of a special value for errors, or splitting a function like getThing(boolean $asString): float|string into two functions getThingAsFloat(): float and getThingAsString(): string.
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