Singleton design pattern

As the name suggests this pattern is related to a notion like " only one of something ".
More specifically , when you want only one instance of a class in your program.
A very common example where this type of design pattern can be applied is when you are doing database connections.
Suppose , if you writing your database connection related code inside a function of a class called DatabaseConn . Now , everytime you create an object of that class and call that function , you are getting a "new"  database connection , which ofcouse is not necessary and is a overhead to the database server. To avoid such problems , you can apply singleton pattern, and ensure that there is only one instance of that class is generated.
The following is an example ( in php ):

	//Class to make db connection
	Class DbConnect {
	//Static member variable to hold the connection object
	private static $conn = NULL;
	private static $instance = NULL;
	private function __construct() {
	// Create connection
	self::$conn = new mysqli("hostname", "username", "password", "dbname");
	// Check connection
	if (self::$conn->connect_error) {
	echo "Connection failed: " . $conn->connect_error;
	}
	}
	public static function connect() {
	if (self::$instance === NULL) {
	self::$instance = new DbConnect();
	return self::$instance;
	} else {
	return self::$instance;
	}
	}
	}
	$myDbConn = DbConnect::connect();
	$myDbConn2 = DbConnect::connect(); //If you var dump these two objects , you will find that they are same instances.
	var_dump($myDbConn);
	var_dump($myDbConn2);