PHP5: when do I close my database object connection?

Hello Experts,

In the sample database code below, when do I close the database obj connection?
I use the object to query the db throughout the session....

//***** index.php 

require_once 'Database.php';

$db = new Database();
$db->getInstance();

$db->testQuery();
$email = $db->testQuery();


//***** Database.php

class Database extends mysqli_stmt {

	//set db access data
	private $server = "localhost";
	private $user = "test";
	private $pass = "test";
	private $db = "test";
	
	private $instance;

	function getInstance(){
	
		$db = new mysqli ( $this->server, $this->db, $this->pass, $this->user );
	
		if ($db->connect_error) {
			die('Connect Error (' . $instance->connect_errno . ') ');
		} else {
			$this->instance = $db;
		}		
	
		return $this->instance;
	}
	
	function testQuery( $name ){
		
		$stmt =  $this->instance->stmt_init();
		
		$query = "SELECT `email` FROM customers WHERE Name=?";
		if ($stmt->prepare($query)) {		
			$stmt->bind_param("s", $name);
			$stmt->execute();
			$stmt->bind_result( $email );
			$stmt->fetch();
			$stmt->close();
		}
		
		return $email;
		
	}
	
	function dbClose(){
		$this->instance->close();
	}
	
	
}//END OF CLASS...

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Regards,