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To translate one interface for a class into a compatible interface. An adapter allows classes to work together that normally could not because of incompatible interfaces by providing its interface to clients while using the original interface.

• DB Client libraries adapter.
• Using multiple different web services and adapters normalise data so that the outcome is the same for all.

Book

namespace DesignPatterns\Structural\Adapter;

interface Book
{
    public function turnPage();
    public function open();
    public function getPage(): int;
}

EBook

namespace DesignPatterns\Structural\Adapter;

interface EBook
{
    public function unlock();
    public function pressNext();

    /**
     * returns current page and total number of pages, like [10, 100] is page 10 of 100
     *
     * @return int[]
     */
    public function getPage(): array;
}

EBookAdapter

namespace DesignPatterns\Structural\Adapter;

/**
* This is the adapter here. Notice it implements Book,
* therefore you don't have to change the code of the client which is using a Book
*/
class EBookAdapter implements Book
{
    public function __construct(protected EBook $eBook)
    {
    }

    /**
    * This class makes the proper translation from one interface to another.
    */
    public function open()
    {
        $this->eBook->unlock();
    }

    public function turnPage()
    {
        $this->eBook->pressNext();
    }

    /**
    * notice the adapted behavior here: EBook::getPage() will return two integers, but Book
    * supports only a current page getter, so we adapt the behavior here
    */
    public function getPage(): int
    {
        return $this->eBook->getPage()[0];
    }
}

Kindle

namespace DesignPatterns\Structural\Adapter;

/**
* this is the adapted class. In production code, this could be a class from another package, some vendor code.
* Notice that it uses another naming scheme and the implementation does something similar but in another way
*/
class Kindle implements EBook
{
    private int $page = 1;
    private int $totalPages = 100;

    public function pressNext()
    {
        $this->page++;
    }

    public function unlock()
    {
    }

    /**
    * returns current page and total number of pages, like [10, 100] is page 10 of 100
    * 
    * @return int[]
    */
    public function getPage(): array
    {
        return [$this->page, $this->totalPages];
    } 
}

PaperBook

namespace DesignPatterns\Structural\Adapter;

class PaperBook implements Book
{
    private int $page;

    public function open(): void
    {
        $this->page = 1;
    }

    public function turnPage(): void
    {
        $this->page++;
    }

    public function getPage(): int
    {
        return $this->page;
    }
}

Tests

public function testCanTurnPageOnBook()
{
    $book = new PaperBook();
    $book->open();
    $book->turnPage();

    $this->assertSame(2, $book->getPage());
}

public function testCanTurnPageOnKindleLikeInANormalBook()
{
    $kindle = new Kindle();
    $book = new EBookAdapter($kindle);

    $book->open();
    $book->turnPage();

    $this->assertSame(2, $book->getPage());
}