PHP Declares Parameter Types

PHP supports type declarations in function signatures. This means you can specify the expected type of parameters and the return type of the function. This helps prevent bugs, makes your code more self-documenting and allows PHP to optimize your code better.

As of PHP 7, the following types can be declared:

bool
int
float
string
array
callable
iterable
object
self
Class and interface names
?type (nullable type)

Here is an example of a function with type declarations:

function add(int $a, int $b): int {
    return $a + $b;
}

In this example, $a and $b must be integers, and the function must return an integer. If you try to pass a different type or return a different type, PHP will throw a TypeError.

In addition to these, PHP 7.1 introduced nullable types, which are specified by prefixing the type name with a question mark (?). Here's an example:

function getFullName(?string $firstName, ?string $lastName): ?string {
    if ($firstName === null || $lastName === null) {
        return null;
    }

    return $firstName . ' ' . $lastName;
}

In this example, $firstName and $lastName can either be strings or null, and the function can either return a string or null.

Finally, as of PHP 8.0, union types are also supported, allowing a parameter or return value to be of multiple types. For example:

function printData(string|bool|int $data): void {
    echo $data;
}

This means the $data parameter can be either a string, bool, or int.

Remember, type declarations are a powerful tool that can help you write more robust and understandable code. But also keep in mind that one of the strengths of PHP is its flexibility, so use type declarations where appropriate, but don't feel like you need to use them everywhere if they don't make sense for your application.

Type hinting in PHP function parameters:
- Type hinting ensures that a function receives the expected data type.
```
function add(int $a, int $b): int {
    return $a + $b;
}
```
Declaring scalar types (int, float, string, bool) in PHP:
- Scalar type declarations restrict function parameters to specific scalar types.
```
function greet(string $name): string {
    return "Hello, $name!";
}
```
Class and interface type declarations in PHP:
- Type hinting with class or interface ensures the parameter is an instance of the specified type.
```
class User {}

function processUser(User $user): void {
    // Process User object
}
```
Nullable types and default values in PHP function parameters:
- Parameters can be nullable or have default values.
```
function display(?string $text = null): void {
    echo $text ?? "No text provided";
}
```

Using array and iterable types in PHP function declarations:

Specify that a parameter should be an array or implement the iterable interface.

function processArray(array $data): void {
    // Process array
}

function processIterable(iterable $data): void {
    // Process iterable
}

Mixing type declarations and variadic parameters in PHP:
- Combine type declarations with variadic parameters.
```
function sum(int ...$numbers): int {
    return array_sum($numbers);
}
```
Benefits of parameter type declarations for code safety:
- Type declarations improve code clarity and prevent unexpected data types.
```
function calculatePrice(float $price, int $quantity): float {
    return $price * $quantity;
}
```

Type coercion and compatibility in PHP function parameters:

PHP performs automatic type conversion, but type declarations enforce strict typing.

function concatStrings(string $str1, string $str2): string {
    return $str1 . $str2;
}

concatStrings("Hello", 123);  // Error without type coercion