php - Passing class from another class or dependency injection -

i'm having difficulties understand given code , reason behind dependency injection. i've got following error:

uncaught error: call undefined method question::getfullname() in c:\xampp\htdocs\oop\index.php:10 stack trace: #0 {main} thrown in c:\xampp\htdocs\oop\index.php on line 10.

even if instantiate object of author class in constructor, keep getting string in question class once try use getquestion().

require 'author.php';  class question {   private $author;   private $question;    public function __construct($question, author $author) {     $this->author = $author;     $this->question = $question;   }    public function getauthor() {     $firstname = $this->author->getfirstname();     $lastname = $this->author->getlastname();     $fullaname = $firstname . $lastname;      return $this;   }    public function getquestion() {     return $this->question;   } }  <?php  class author {   private $firstname;   private $lastname;   private $fullname;    public function __construct($firstname, $lastname) {     $this->firstname = $firstname;     $this->lastname = $lastname;   }    public function getfirstname() {     return $this->firstname;   }    public function getlastname() {     return $this->lastname;   }    public function getfullname() {     return $this->fullname = $this->firstname." ".$this->lastname;   } } 

require 'question.php';  $question = new question("what author's name?", new author("josel", "parayno"));  echo $question->getquestion(); echo $question->getfullname(); 

$question not have getfullname method. method getfullname exists in class author. , after creating , "sending" question, when created method getfullname available in class question private $author property.

but if want athor name follow code, need try

$question->getauthor()->getfullname(); 

and if this, take error again, becouse in question->getauthor return $this, , in case question object. getting author name question object should follow:

1. fix getauthor this

public function getauthor()  {     $firstname = $this->author->getfirstname();     $lastname = $this->author->getlastname();     return $this->author; } 

2. rewite call name in index.php this

echo $question->getauthor()->getfullname();