PHP Type Hints

Summary: in this tutorial, you’ll learn how to use PHP type hints to declare the types for function parameters and return values.

Introduction to PHP type hints

PHP is a dynamically typed language. When you define a function, you don’t need to declare types for parameters. For example:

<?php function add($x, $y) { return $x + $y; } $result = add(1,2); echo $result; // 3
Code language: PHP (php)

The add() function accepts two arguments and returns the sum of them. In this example, we pass two integers into the add() function and get the result as an integer.

If you pass two floating-point numbers into the add() function, you’ll get the sum of the floats, which is a floating-point number:

<?php function add($x, $y) { return $x + $y; } $result = add(1.0,2.5); echo $result; // 3.5
Code language: PHP (php)

More interestingly, you can pass an integer and a numeric string into the add() function, it will return an integer:

<?php function add($x, $y) { return $x + $y; } $result = add(1,'2'); echo $result; // 3
Code language: PHP (php)

In this case, the add() function implicitly coerces the numeric string '2' into the integer 2 because of the + operator. If PHP fails to coerce the string argument into an integer, it’ll issue an error. For example:

<?php function add($x, $y) { return $x + $y; } $result = add('Hi','There'); echo $result;
Code language: PHP (php)

Error:

Fatal error: Uncaught TypeError: Unsupported operand types: string + string in ...
Code language: PHP (php)

To enforce the types for function parameters and return value, you can use type hints.

Note that PHP also allows you to use type hints for class properties and methods which you’ll learn in the PHP object-oriented programming tutorial.

PHP type hints for function parameters

The type hints ensure that PHP will check the type of a value at the call time and throw a TypeError if there is a mismatch.

To add a type hint to a parameter, you place a type in front of it like this:

<?php function my_function(type $param1, type param2, ...) { // ... }
Code language: PHP (php)

In PHP 5, you can use array, callable, and class for type hints. In PHP 7+, you can also use scalar types such as bool, float, int, and string.

The following defines the add() function that accepts two integers:

<?php function add(int $x, int $y) { return $x + $y; } $result = add(1,2); echo $result; // 3
Code language: PHP (php)

However, if you pass two floats, you’ll get the result as an integer:

<?php function add(int $x, int $y) { return $x + $y; } $result = add(1,2.5); echo $result; // 3
Code language: PHP (php)

In this case, PHP implicitly coerces 2.5 into an integer (2) before calculating the sum. Therefore, the result is an integer.

By default, PHP coerces a value of the compatible type into the expected scalar type declaration if possible. To enforce the value with a type that matches the type declaration, you need to declare strict typing.

PHP type hints for function’s return value

To specify a return value’s type for a function, you add the type after the function header like this:

<?php function my_function(type $param1, type $param2, ...) : type { // .. }
Code language: PHP (php)

The following example defines the add() function that accepts two integers and returns an integer:

<?php function add(int $x, int $y): int { return $x + $y; } echo add(10, 20);
Code language: PHP (php)

Starting from PHP 7.0, if a function doesn’t return a value, you use the void type. For example:

<?php function dd($data):void { echo '<pre>'; var_dump($data); echo '</pre>'; die; }
Code language: PHP (php)

The union type

Starting from PHP 8.0, if a function returns a value of several types, you can declare it as a union type. For example:

<?php function add($x, $y): int | float { return $x * $y; } echo add(10, 20); // 200 (int) echo add(1.5, 2.5); // 3.75 (float)
Code language: PHP (php)

In this example, the add() function returns an integer or a floating-point number, depending on the types of arguments.

The mixed type

If a function returns a value of many types, you can use the mixed type. The mixed type means one of several types. The mixed type. It’s equivalent to the following union type:

object|resource|array|string|int|float|bool|null
Code language: PHP (php)

The mixed has been available since PHP 8.0.0.

For example, the filter_var() built-in function use both union type (array|int) and mixed type as the type hints:

filter_var(mixed $value, int $filter = FILTER_DEFAULT, array|int $options = 0): mixed
Code language: PHP (php)

The nullable type

The following defines a function that accepts a string and returns the uppercase of that string:

<?php function upper(string $str): string { return strtoupper($str); }
Code language: HTML, XML (xml)

If you pass an argument with null, you’ll get an error:

<?php function upper(string $str): string { return strtoupper($str); } $str = null; echo upper($str);
Code language: HTML, XML (xml)

Error:

Fatal error: Uncaught TypeError: Argument 1 passed to upper() must be of the type string, null given
Code language: plaintext (plaintext)

To fix this, you can make the $str parameter nullable like this:

<?php function upper(?string $str): string { return strtoupper($str); } $str = null; echo upper($str);
Code language: HTML, XML (xml)

The nullable type was introduced in PHP 7.1.

PHP allows you to mark the type declarations and returns values as nullable by prefixing the type name with a question mark (?).

In the above example, we add the ? to the string type of the $str parameter. The ?string allows you to pass a string argument or null.

Note that the mixed type already includes the null type. Therefore, you don’t need to include nullable mixed like this:

? mixed

Also, doing so will result in an error.

Summary

  • Use PHP type hints for function parameters and return types.
  • Use void type if the function doesn’t return any value.
  • Use mixed type or union type if function parameter or function return value expect on of several types.
  • To make a type nullable, prefix the type with a question mark (?type).
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