test the time that book has been loaned.

after.php

<?php
class Loan
{
    private $book;
    private $time;
    private static $faketime;

    public function __construct(Book $book)
    {
        $this->book = $book;
        $this->time = self::$faketime ? : new DateTime();
    }
    public static function setFaketime(DateTime $faketime)
    {
        self::$faketime = $faketime;
    }

    public function toString()
    {
        return $this->book->title() . ' loaned on ' . $this->time->format('Y/m/d H:i:s');
    }
}

class Book
{
    private $title;
    public function __construct($title) { $this->title = $title; }
    public function title() { return $this->title; }
}


Loan::setFaketime(new DateTime('2013/06/10 18:00:00'));
$l = new Loan(new Book('WEEKLY JUMP'));
echo $l->toString(); // always returns 'WEKLY JUMP loaned on 2013/06/10 18:00:00'

bad.php

<?php
class Loan
{
    // ....
    
    public function __construct($book, $time)
    {
        $this->book = $book;
        $this->time = $time;
    }

    // ....
}

before.php

<?php
class Loan
{
    private $book;
    private $time;

    public function __construct($book)
    {
        $this->book = $book;
        $this->time = new DateTime();
    }

    public function toString()
    {
        return $this->book->title() . ' loaned on ' . $this->time->format('Y/m/d H:i:s');
    }
}

too_bad.php

<?php
class Loan
{
    // ....
    
    public function __construct($book, $time, $num)
    {
        $this->book = $book;
        $this->time = $time;
        $this->num  = $num;
    }

    // ....
}