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What are "union types" and how do you use them in PHP?

Abel Lifaefi Mbula

In this shot, I want you to learn about a PHP feature called “union type” and how to implement or use it both in PHP 7 and PHP 8. Without any further ado, let’s get started.

What is a union type?

Generally speaking, union types are defined as follows:

A collection of values stored within the same position in memory with different data types/formats.

In short, union type allows you to define a collection of two or more types (int, float…) for your variables or function parameters. For example, you can have a function in your program that accepts a parameter of the int or string type.

The union type’s typical syntax looks like this:

public function foo(Foo|Bar $input): int|float;

The foo function (above) accepts a parameter, $input, of type both Foo and Bar, and returns a value of either an int or float.

Union type in PHP 7

PHP 7 only supports these two special union types:

  • Nullable union types using the ?Type notation: public function bar(?Bar $bar): void;
  • Array or Traversable using the iterable type

PHP 7 does not support arbitrary union types, but as a workaround, we can use phpdoc annotation. Let’s see an example of its use below:

<?php
class Student 
{
    /**
     * @var int|float $roomNo
     */
    private $roomNo;
    /**
     * @param int|float $roomNo
     */
    public function setRoomNo($roomNo) {
        $this->roomNo = $roomNo;
    }
    /**
     * @return int|float
     */
    public function getRoomNo() {
        return $this->roomNo;
    }
}
 
$newStudent1 = new Student;
$newStudent1->setRoomNo(25.0);
$newStudent2 = new Student;
$newStudent2->setRoomNo(4);
 
echo "Student 1 number is " .$newStudent1->getRoomNo(). "<br>";
echo "Student 2 number is " .$newStudent2->getRoomNo();
Class Student

Now, let’s look at how “union type” is improved in PHP 8.

Union type in PHP 8

As we’ve seen above, PHP 7 lacks built-in support for arbitrary union types. So, PHP 8 has implemented built-in support for arbitrary union types.

With the same example as above, here’s how you can make use of union types in PHP 8:

<?php
class Student 
{
    private int|float $roomNo;
    public function setRoomNo(int|float $roomNo): void {
        $this->roomNo;
    }
    public function getRoomNo(): int|float {
        return $this->roomNo;
    }
}
$newStudent1 = new Student;
$newStudent1->setRoomNo(25.0);
$newStudent2 = new Student;
$newStudent2->setRoomNo(4);
 
echo "Student 1 number is " .$newStudent1->getRoomNo(). "<br>";
echo "Student 2 number is " .$newStudent2->getRoomNo();
Class Student (PHP 8)

Note

  • the void type can never be part of a union type because it indicates that the function does return any value
  • nullable union type can also be written as: public function foo(Foo|null $foo): void;

Conclusion

Union types are a great functionality in PHP that let us define a variable of multiple types in our program. PHP 7 supports arbitrary union types through phpdoc annotation, whereas PHP 8 has built-in support for it.

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