Union types in PHP 8 allow variables to accept or return multiple types. They are declared using the | symbol between types, such as int|string, and can be used for function parameters, return values, and class properties (as of PHP 8.2). For example, a function can return either an integer or string with function getStatusCode(): int|string. Union types improve type safety while offering flexibility over loose typing or comments. They should be used when a value logically supports multiple types, like optional null returns (User|null) or handling different input formats (string|array). Avoid using union types when unsure—use mixed instead. Inside functions, use is_*() or instanceof checks to handle logic based on the actual type, ensuring safer and clearer code execution.

Union types in PHP 8 allow a function or method parameter, return value, or class property to accept or return more than one type. This means instead of restricting a variable to a single type like int or string, you can specify that it can be either one type or another.

For example, if a function can return either an integer or a string depending on context, you can now explicitly declare that with union types:

function getStatusCode(): int|string {
    // returns either an int or a string
}

This feature improves type safety while giving developers more flexibility without relying on comments or loose typing.

Declaring Union Types

You declare union types by separating the allowed types with a vertical bar (|). This syntax works for parameters, return types, and properties:

• Function parameters:

  function logData(string|int $data): void {
      // accepts either a string or an int
  }

• Return types:

  function fetchValue(): float|bool {
      // returns either a float or a boolean
  }

• Class properties (PHP 8.2 ):

  class Example {
      public string|int $value;
  }

If the value doesn’t match any of the specified types at runtime, PHP will throw a TypeError.

When to Use Union Types

Use union types when a value logically has more than one valid type. Common scenarios include:

• Optional values that might be null

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

• Functions that handle multiple input formats

  function parseInput(string|array $input): array {
      // accepts a JSON string or an array
  }

• Mixed data structures where strict typing isn't practical but some level of control is still needed

Avoid using them just because you're unsure of the type — that's what mixed is for. Use union types when you know exactly which types are acceptable.

Handling Type Checks

Even though PHP enforces union types at runtime, you’ll often want to check the actual type inside your function logic. You can use is_*() functions or instanceof as needed:

function processValue(int|string $value): void {
    if (is_int($value)) {
        echo "Got an integer: $value";
    } else {
        echo "Got a string: '$value'";
    }
}

Some tips:

• Always validate the type before performing operations specific to one type
• Consider returning early or throwing exceptions if an unexpected type slips through
• Use match or get_debug_type() in more complex cases to improve readability

Type checks help avoid bugs and make your code clearer, especially when different logic applies based on the actual type.

That’s basically how union types work in PHP 8. They give you more precision without losing flexibility, and they’re pretty straightforward once you get used to the syntax.

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