PHP design patterns - inheritance vs adapter pattern?

I am a bit confused with the concept of adapter pattern. I find that adapter classes are very similar to extended classes that I would write usually. So, what is the differences between them actually?

For instance (an example from this link),

SimpleBook.php,

class SimpleBook {


    private $author;
    private $title;


    function __construct($author_in, $title_in) {
      $this->author = $author_in;
      $this->title  = $title_in;
    }


    function getAuthor() {return $this->author;}


    function getTitle() {return $this->title;}


  }

BookAdapter.php

include_once('SimpleBook.php');

  class BookAdapter {


    private $book;


    function __construct(SimpleBook $book_in) {
      $this->book = $book_in;
    }


    function getAuthorAndTitle() {
      return $this->book->getTitle() . ' by ' . $this->book->getAuthor();
    }


  }

BookExtension.php,

include_once('SimpleBook.php');

      class BookExtension extends SimpleBook{

        function getAuthorAndTitle() {
          return $this->getTitle() . ' by ' . $this->getAuthor();
        }


      }

The second solution seems a lot simpler. So is it (and other inheritance classes in general) considered as adapter class then?