How to connect to OOP DB?

Question

PHP

I'm trying to do so

class connect{
    public $db = mysqli_connect('localhost', 'root', 'amV5TbDYH8', 'News');
    public function dbconnect($db){
      if($db){
        echo "ok";
      } die (mysql_error());
    }
  }

but because of the line

public $db = mysqli_connect('localhost', 'root', 'amV5TbDYH8', 'News');

The site crashes and writes
Page not available
Site ***** cannot process this request yet.