PHP 8.0 introduces joint types to improve type prompt flexibility. 1. The joint type uses | symbols to declare variables, parameters or return values ??to accept multiple types, such as string|int; 2. Solve the problem of relying on mixed or annotations before, enhance runtime type checking and improve IDE support; 3. Support nullable values ??such as User|null to clearly express possible missing data; 4. Allow functions to accept multiple input formats such as string|ContentData to improve flexibility and maintain type safety; 5. Compared with mixed and object, joint types are more specific and have a wider range of applications; 6. Pay attention to type compatibility and logical rationality when using to avoid excessive use. The union type makes the code clearer, safer, and easier to maintain.

PHP 8.0 introduced a powerful feature called union types , which significantly enhances type hinting flexibility by allowing developers to specify that a parameter, return value, or class property can be one of several types.

What Are Union Types?

In PHP, union types let you declare that a variable, function parameter, or method return can be more than one type. You express this using the | (pipe) character between type names.

For example:

 function renderText(string|int $input): void {
    echo $input;
}

Here, $input is allowed to be either a string or an int . This eliminates the need for comments or loose typing when multiple types are valid.

Why Use Union Types?

Before PHP 8.0, if a function accepted multiple types, you had to omit type hints entirely or use docblocks to document the possible types. That made code less strict and harder to understand at a glance.

With union types:

• Type expectations are enforced at runtime.
• IDEs and static analyzers can provide better autocomplete and error checking.
• You avoid relying on mixed , which is too broad and not describe.

This leads to cleaner, safer, and more maintainedable code.

How Do Union Types Improve Flexibility?

Union types give you fine-grained control over what types are acceptable without sacrificing type safety. Here are some practical scenarios:

? Accepting Nullable Values ??with null

You can now clearly express that a value might be absent:

 function findUser(int $id): User|null {
    // returns User object or null
}

This is clearer than using ?User , which is still valid but just syntactic sugar for null|User .

? Supporting Multiple Input Formats

Imagine a function that accepts either a raw string or a preprocessed data object:

 function parseContent(string|ContentData $data): array {
    // process based on type
}

Now, both strings and ContentData objects are valid inputs, and it's enforced by the type system.

? Handling Optional Parameters

When designing APIs or libraries, sometimes parameters can vary in type depending on context:

 function formatValue(float|int|string $value): string {
    // formats differently based on input type
}

This makes your function more flexible while keeping strong typing intact.

Union Types vs. mixed and object

Before union types, developers often used mixed as a fallback, but that's essentially a wildcard — anything goes, and no guaranteees are made. Union types offer specificity without being overly restrictive.

Also, using object was another workaround, but that only covers objects, not primitives like int or string .

Some Things to Keep in Mind

While union types are powerful, there are a few gotchas:

• Order doesn't matter: string|int is the same as int|string .
• You can't combine incompatible types like array|Traversable unless they're logically related.
• You can't use false or true as standalone types — only bool works.
• Union types don't replace polymorphism; they're best for simple variations.

So use them where appropriate — not every function needs to accept five different types.

Basically that's it. Union types are a straightforward way to make your type hints more expressive and accurate without losing clarity or safety. They fit right into modern PHP development and help reduce ambiguity in APIs, libraries, and applications.

The above is the detailed content of What are union types in PHP 8.0, and how do they improve type hinting flexibility?. For more information, please follow other related articles on the PHP Chinese website!