PHP OOP Database class using MySQLI and Singleton pattern. Only one instance of the class will be made, this requires less memory.

class.database.php

<?php
/*
* Mysql database class - only one connection alowed
*/
class Database {
	private $_connection;
	private static $_instance; //The single instance
	private $_host = "HOSTt";
	private $_username = "USERNAME";
	private $_password = "PASSWORd";
	private $_database = "DATABASE";

	/*
	Get an instance of the Database
	@return Instance
	*/
	public static function getInstance() {
		if(!self::$_instance) { // If no instance then make one
			self::$_instance = new self();
		}
		return self::$_instance;
	}

	// Constructor
	private function __construct() {
		$this->_connection = new mysqli($this->_host, $this->_username, 
			$this->_password, $this->_database);
	
		// Error handling
		if(mysqli_connect_error()) {
			trigger_error("Failed to conencto to MySQL: " . mysql_connect_error(),
				 E_USER_ERROR);
		}
	}

	// Magic method clone is empty to prevent duplication of connection
	private function __clone() { }

	// Get mysqli connection
	public function getConnection() {
		return $this->_connection;
	}
}
?>

I feel like i should know this stuff already I think i have used it before but right now i have had some time away from coding due to life happening and now i can't seem to get my brain to cooperate with what i am trying to get done,
so I started putting together a simple user system and with a simple connection everything worked fine once i changed to this DB class file and included it simple things became broken and i just can not seem to work it out so my original sql select query was a prepared statement,
$sql = "SELECT * FROM users WHERE username=? AND password=? AND user_type=?"; $stmt = $mysqli->prepare($sql); $stmt->bind_param("sss",$username,$password,$userType); $stmt->execute(); $result = $stmt->get_result(); $row = $result->fetch_assoc();

this was working just fine with the standard inline connection, now with the class file included i get

"Fatal error: Uncaught Error: Call to a member function prepare() on null in /homepages/15/d842734408/htdocs/MURB/index.php:22 Stack trace: #0 {main} thrown in"

can anyone please correct my line of thinking on this so i can move on and remember what i need here please