Union type in PHP8.0

With the release of PHP 8.0, a new type - union type (also called "union type") was introduced, which can be used to declare that a variable can be one of multiple types.

In many object-oriented languages, we can use the concept of polymorphism to define that a variable can be an instance of its own type, or its subtype or implemented interface. Any instance. But in PHP, we have only been able to achieve this goal by putting several types of conditional statements together, including in switch, if, and elseif conditional blocks. This approach is sometimes cumbersome and difficult to maintain.

Therefore, PHP8.0 adds a new union type to achieve polymorphism that gets rid of conditional statements. In this article, we will introduce the definition, usage, syntax and some considerations of the union type.

1. Define union type

When defining a union type variable, you need to separate the names of several types with vertical bars (|), for example:

function foo(int|float|string $param) {}

This defines a parameter $param, which can be one of integer, floating point or string types.

Normally, we can also use the null keyword to specify that a variable of union type can be a null value:

function bar(string|null $param) {}

This means that $param can have two different types: string or null.

However, union types do not support recursive definitions. That is to say, we cannot use the following syntax:

function baz(int|float|array $param, string|null|float $other) {}

2. Use union type

It is very convenient to use union type in functions, method parameters and return types, properties, etc. For example:

// 函数参数中使用union类型：
function test(int|float $param) {}

// 方法返回类型中使用union类型：
class Foo {
    public function bar(): string|array {}
}

// 属性类型中使用union类型：
class Baz {
    public int|float $foo;
}

However, using union types in variable declarations is more restrictive because they can only be assigned to a value of one of their types.

// 变量声明中使用union类型：
$test = 1|2.0|"three";
$test = "four";  // 可以赋值为其中一个类型的值
$test = false;  // 不是其中任何一个类型，将会抛出TypeError

We can use is_int(), is_float(), is_string() and other functions to check union type variables.

3. Notes

Although the union type can reduce maintenance and repeated code in various scenarios, it also has some things to pay attention to.

First of all, we need to use the union type with caution, because due to its flexibility, it may increase the complexity of the code. If there are too many union types in your code, it may mean that your code may have a design problem, especially with the parameters and return types of functions and methods.

Secondly, although the union type is a new change in PHP8.0, it is not supported by all versions. If you want to use the union type, you need to make sure that your code is running PHP8.0 and above, otherwise the code will not run properly.

Finally, we need to pay attention to the syntax of using union type. The correct syntax is to separate multiple type names with a pipe (|), such as int|float|string, not int|float|string or int|float|string.

4. Summary

In PHP8.0, the new union type provides a more flexible and clear solution for our code when dealing with complex type judgments. played a big role. Although it conforms to the concept of polymorphism, we must also pay attention to its use to avoid overuse and negative impact on the code. If you are considering upgrading to PHP8.0, you might as well try using the union type, maybe it will be a good helper for you.

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