Challenge 1 — Solution

Task: Create a Book class with public properties $title, $author, and a
constructor that sets both. Add a method describe() that echoes
"[title] by [author]". Create two different Book objects and call
describe() on each.

<?php
    class Book {
        public string $title;
        public string $author;

        public function __construct(string $title, string $author) {
            $this->title = $title;
            $this->author = $author;
        }

        public function describe() {
            echo $this->title . " by " . $this->author . "<br>";
        }
    }

    $book1 = new Book("The Pragmatic Programmer", "David Thomas");
    $book2 = new Book("Clean Code", "Robert C. Martin");

    $book1->describe();
    $book2->describe();
?>


Output:
The Pragmatic Programmer by David Thomas
Clean Code by Robert C. Martin

Notes:
- The constructor runs automatically the moment "new Book(...)" is
  called, immediately assigning both properties - no separate line is
  needed to set $title or $author afterward.
- describe() reads $this->title and $this->author, referring to
  whichever object it was called on - the exact same method body
  produces two different results because $this points at a different
  object each time.
- $book1 and $book2 are two completely independent objects, each with
  its own copy of $title and $author, even though both were created
  from the identical Book blueprint.