Sensible Interfaces

libraryFile.php

<?php

namespace SrcFolder\LibraryNamespace

interface LibraryInterface
{
    public addItemMethod(ItemInterface $itemArgument)
    public rentMethod(ItemInterface $itemArgument, ClientInterface $clientArgument);
}

interface ItemInterface
{
    public getIdentifierMethod();
}

interface ClientInterface
{
    public getIdentifierMethod();
}

class LibraryClass implements LibraryInterface
{
    private $inventoryProperty = [];
    private $rentsProperty = [];
    
    public function addItemMethod(ItemInterface $itemArgument)
    {
        //...
    }
    
    public function rentItemMethod(ItemInterface $itemArgument, ClientInterface $clientArgument)
    {
        //...
    }
}

class BookClass implements ItemInterface
{
    private $isbnProperty;
    
    //...
}