PHP Type Declarations: Typed Parameters and Return Values

A type declaration tells PHP what kind of value a parameter must be. It catches mistakes early and makes functions easier to understand.

index.php

PHP

<?php
function repeatText(string $text, int $times): string
{
    return str_repeat($text, $times);
}

echo repeatText("Ha", 3);

Output

OUTPUT

HaHaHa

string $text means $text must be a string. int $times means a whole number.

Types you can use

Type	Meaning
`int`	Whole number
`float`	Decimal number
`string`	Text
`bool`	`true` or `false`
`array`	An array
`callable`	Something that can be called
`iterable`	An array or anything you can `foreach`
`object`	Any object
`mixed`	Anything at all
`ClassName`	An object of that class

What happens with the wrong type

By default, PHP converts a value if it can ("5" becomes 5) and throws a TypeError if it cannot.

index.php

PHP

<?php
function double(int $n): int
{
    return $n * 2;
}

echo double("5") . "\n";   // converted

try {
    echo double("abc");   // cannot be converted
} catch (TypeError $e) {
    echo "TypeError: " . $e->getMessage();
}

Output

OUTPUT

1
2

10
TypeError: double(): Argument #1 ($n) must be of type int, string given, called in /path/to/index.php on line 10

Add declare(strict_types=1); to stop conversions as well. See Strict Typing.

Nullable types

A ? before the type allows null too.

index.php

PHP

<?php
function greet(?string $name): string
{
    return "Hello, " . ($name ?? "guest");
}

echo greet("Mira") . "\n";
echo greet(null);

Output

OUTPUT

1
2

Hello, Mira
Hello, guest

Union types (PHP 8.0+)

A parameter can accept more than one type, separated by |.

index.php

PHP

<?php
function showId(int|string $id): string
{
    return "ID: " . $id;
}

echo showId(42) . "\n";
echo showId("A-17");

Output

OUTPUT

1
2

ID: 42
ID: A-17

Class types

index.php

PHP

<?php
class Product {}

function describe(Product $product): string
{
    return "A product object";
}

echo describe(new Product());

Output

OUTPUT

A product object

Why you should use them

Errors show up at the function call, not three steps later.
Your editor can warn you about wrong calls.
Anyone reading the function sees what it needs without a comment.

lightbulb

Type everything you write

Add types to every parameter and return value in new code. It costs a few seconds and saves hours of debugging.