Warning: mysqli_select_db() expects parameter 1

<?php
final class MySQL {
	private $connection;
	
	public function __construct($hostname, $username, $password, $database) {
		if (!$this->connection = mysqli_connect($hostname, $username, $password)) {
      		exit('Error: Could not make a database connection using ' . $username . '@' . $hostname);
    	}

    	if (!mysqli_select_db($mysqli_connection, '$database')) {
      		exit('Error: Could not connect to database ' . $database);
    	}
		
		mysqli_query($this->connection, "SET NAMES 'utf8'");
		mysqli_query($this->connection, "SET CHARACTER SET utf8");
		mysqli_query($this->connection, "SET CHARACTER_SET_CONNECTION=utf8");
		mysqli_query($this->connection, "SET SQL_MODE = ''");
  	}