如何在PHP中从外部文件创建对象

我在一个类中有一个mySQL对象，它在一个外部文件中（从其他几个文件访问它）。如何从文件dbconnection.php中获取Singleton对象：

<?php
class DBConnection {
  private static $instance;
  private function __construct() {
    $user="root";
    $password="";
    $database="klb";
    mysql_connect("localhost",$user,$password);
    @mysql_select_db($database) or die( "Unable to select database");  
 }
  public function __destruct() {
      mysql_close();
 }
  public static function getInstance() {
    if(!self::$instance) {
      self::$instance = new self();
    }
    return self::$instance;
  }
  public function getNaviForCategory($category) {
    $query="SELECT * FROM projects WHERE category='"$category'"";
    $result=mysql_query($query);
    $num=mysql_numrows($result);
    while ($row = mysql_fetch_object($result)) {
        $title=$row->title;
        $id=$row->id;
        echo "<div class='"sublink'" data-subsite='"$id'" data-category='"$category'" data-title='"$title'" ><a href='"#'">$title<br />";
    }
  }
 public function getInfosForProject($id) {
    $query="SELECT * FROM projects WHERE id='"$id'"";
    $result=mysql_query($query);
    $num=mysql_numrows($result);
    while ($row = mysql_fetch_object($result)) {
        $infos=$row->info;
        echo $infos;
    }
  } 
   private function createTableProjects(){
        $query="CREATE TABLE projects (id int(6) NOT NULL auto_increment,category varchar(30) NOT NULL,title varchar(30) NOT NULL,
        info varchar(200) NOT NULL,text varchar(8000) NOT NULL,PRIMARY KEY (id),UNIQUE id (id))";
        mysql_query($query);
   }
}
?>