解釋一個數據庫存分類

woff

private $db_host; //數據庫主機
private $db_user; //數據庫用戶名
private $db_pwd; //數據庫用戶名密碼
private $db_database; //數據庫名
private $conn; //數據庫連接標識;
private $coding; //數據庫編碼，GBK,UTF8,gb2312
public function __construct($db_host, $db_user, $db_pwd, $db_database, $conn, $coding) {
  $this->db_host = $db_host;
  $this->db_user = $db_user;
  $this->db_pwd = $db_pwd;
  $this->db_database = $db_database;
  $this->conn = $conn;
  $this->coding = $coding;
  $this->connect();
}
/*數據庫連接*/
public function connect() {
  if ($this->conn == "pconn") {
   //永久鏈接
   $this->conn = mysql_pconnect($this->db_host, $this->db_user, $this->db_pwd);
  } else {
   //即使鏈接
   $this->conn = mysql_connect($this->db_host, $this->db_user, $this->db_pwd);
  }
  if (!mysql_select_db($this->db_database, $this->conn)) {
   if ($this->show_error) {
    $this->show_error("數據庫不可用：", $this->db_database);
   }
  }
  mysql_query("SET NAMES $this->coding");
}

$db = new action($mydbhost, $mydbuser, $mydbpw, $mydbname, ALL_PS, $mydbcharset);

這是MYSQL分類,我不明白永久聯接if ($this->conn == "pconn") 這條語句是什麼時意思呀,CONN為數據庫連接標識;初始化時給了賦了一個值ALL_PS,這個ALL_PS是什麼呀,是自定義變量,但是我沒有查文件中看到ALL_PS有什麼值賦給他,請高手回答一下,
還有就是永久連接和即使連接有什麼區別呀?????


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