Undefined variable in php constructor function

Undefined variable $title at this code don’t know what’s the problem to it am a beginner to php

   Class Book{
     
    var $title;   //attribute
  
  function __construct($Atitle){

   $this ->$title = $aTitle; // constructor variables
}
}
$book1 = new book("Harry Potter");
echo $book1->$title;