The document discusses limited return type covariance and argument type contravariance in PHP. It provides examples of how a child class can override a parent method to change the return type to a more specific subtype and how a child class can override a constructor to make argument types more generic than the parent. It also includes code examples demonstrating mapping array values, spreading array elements into another array, and defining basic classes.

1. Limited return type covariance
and
argument type contravariance

2. ☺

3. 🎉🎉

4. ✋

7. <?php
class User {
public int $id;
public string $name;
}
?>

8. <?php
$factor = 10;
$nums = array_map(
fn($n) => $n * $factor, [1, 2, 3, 4]
// function($n) use ($factor){
// return $n * $factor;
// } ,[1, 2, 3, 4]
);
// $nums = array(10, 20, 30, 40);
?>

9. <?php
$parts = ['apple', 'pear'];
$fruits = ['banana', 'orange', ...$parts ,
‘watermelon'];
// ['banana', 'orange', 'apple', 'pear', 'watermelo
n'];
?>

11. Limited return type covariance
and
argument type contravariance

13.
🎉🎉

16. 🎉🎉

21.
<?php
class A {}
class B extends A {}
class Producer {
public function method(): A {}
}
class ChildProducer extends Producer {
public function method(): B {}
}
?>

28.
<?php
class A {}
class B extends A {}
class Producer(B $b) {
public function method(): A {}
}
class ChildProducer(A $a) extends Producer {
public function method(): B {}
}
?>

36. 😚

37. 👿