php : storing reference to other class object

php : storing reference to other class object

I come from a java background and m trying a hand at php. Right now I m
trying to pass a object to constructor of a class and trying to store a
reference to it inside the class and upon a function call to this call
execute a method from the stored reference.
$phpBook = new Book("Php Book", 500);
$vihaan = new Person("Vihaan", $phpBook);
Person.php
class Person
{
private $_book;
private $_name;
public function __construct($name, $book)
{
$_this->_book = $book;
$_this->_name = $name;
}
on this line
$_this->_book = $book;
I get a warning.
PHP Warning: Creating default object from empty value in
/home/vihaan/workspace/AdapterPattern1/Person.php on line 12
and this function call never enter the if block as $_book seems to be empty.
public function openBook($pageNumber = 0)
{
if(!empty($_book))
{
$_book->open($pageNumber);
}
}