Database class design - php

Question

I'm creating a web app with various classes for things like the user, Smarty template control, etc.
I already have a database class which is all well and good, but I'm concerned about the performance of it.
Currently, in another class, I'm doing $this->db = new DB() to create a local database instance, however the database class's __construct() function creates a new connection to the MySQL server every time I make a new DB() instance, which is obviously less than sensible. This means that each instance of all my different classes that uses the database class makes a connection to the server. I don't have a vast amount of classes, but I only want one per page load.
This is a stripped down sample of what I have at the moment:
// Database class used by multiple other classes
class DB {
private $dbh;
function __construct() {
$this->dbh = // PDO connection here
}
public function query($str) {
// Do a query
}
}
// Example class User
class User {
private $db; // Stores local instance of DB class.
function __construct() {
$this->db = new DB(); // Makes a new connection in DB::__construct()
}
public function login() {
$this->db->query('SELECT * FROM users');
}
}
I'm looking for the "best" or most common practice of doing this. I don't want to make 10-ish separate connections for each page load.
I want to know what the best way of using and managing a DB class in my application. My four thoughts are these:
Would using a persistent connection to the MySQL server solve this multiple connection issue for me?
Should I use a static factory class and return a DB instance instead of using new DB()?
Is the proper solution to use an entirely static class and just do DB::query() (for example) every time I reference it?
I often use multiple classes in another (so we might have class Folders which requires classes User, DB and Smarty). Is it general practice to extend each class somehow?

Answer 1

If you make the variable holding the connection static, then you can check if you already established a connection. Static variables are the same across all instances of the class, so you can create 100 instances that all use the same connection. You just need to reference it statically: self::$dbh instead of $this->dbh.
class DB {
private static $dbh = null;
function __construct() {
if ( is_null(self::$dbh) ) {
self::$dbh = // PDO connection here
}
}
}

Answer 2

I would suggest you to check the $this -> db at first and then only create it.
function __construct() {
if(!isset($this -> db) || !is_a("DB", $this -> db)) {
$this->db = new DB(); // Makes a new connection in DB::__construct()
}
}

Answer 3

You need to inject db connection to your class instead of creating a new connection.
// In a bootstrap file
$db = new DB();
// User.php
class User {
private $db;
function __construct($db=null) {
if (!is_null($db)) {
$this->setConnection($db);
}
}
function setConnection($db) {
$this->db = $db;
}
public function login() {
$this->db->query('SELECT * FROM users');
}
}
BTW, Zend_Registry is a good solution if you prefer it http://framework.zend.com/manual/en/zend.registry.using.html

Answer 4

<?php
class DBLayer {
public $prefix;
public $link_id;
public $query_result;
public $saved_queries = array();
public $num_queries = 0;
public function DBLayer() {
$db_prefix = '';
$this->prefix = $db_prefix;
if (isset($this->link_id)) {
return $this->link_id;
}
$this->link_id = #mysql_connect(DATABASE_HOST, DATABASE_USER, DATABASE_PASSWORD, true);
if ($this->link_id) {
if (#mysql_select_db(DATABASE_NAME, $this->link_id)) {
return $this->link_id;
} else {
$this->wplog("Unable to select database. Host:". DATABASE_HOST. "Database:" . DATABASE_NAME . " Error: " . mysql_error(), 'ERROR', __FILE__, __LINE__);
}
} else {
$this->wplog("Unable to connect to MySQL server. Host: " . DATABASE_HOST . " Error: " . mysql_error(), 'ERROR', __FILE__, __LINE__);
}
}
public function query($sql, $unbuffered = false) {
if(LOG){echo "<hr>$sql";}
$this->query_result = #mysql_query($sql, $this->link_id);
if ($this->query_result) {
return $this->query_result;
} else {
$msg= $sql . "<br /> Error: (" . mysql_errno() . ") " . mysql_error();
$this->wplog($msg);
}
}
public function result($query_id = 0, $row = 0) {
return ($query_id) ? #mysql_result($query_id, $row) : false;
}
public function fetch_assoc($query_id = 0) {
return ($query_id) ? #mysql_fetch_assoc($query_id) : false;
}
public function fetch_row($query_id = 0) {
return ($query_id) ? #mysql_fetch_row($query_id) : false;
}
public function num_rows($query_id = 0) {
return ($query_id) ? #mysql_num_rows($query_id) : false;
}
public function affected_rows() {
return ($this->link_id) ? #mysql_affected_rows($this->link_id) : false;
}
public function insert_id() {
return ($this->link_id) ? #mysql_insert_id($this->link_id) : false;
}
public function get_num_queries() {
return $this->num_queries;
}
public function get_saved_queries() {
return $this->saved_queries;
}
public function free_result($query_id = false) {
return ($query_id) ? #mysql_free_result($query_id) : false;
}
public function escape($str) {
if (function_exists('mysql_real_escape_string'))
return mysql_real_escape_string($str, $this->link_id);
else
return mysql_escape_string($str);
}
public function get_select($q, $onlyone=false) {
$results = array();
$r = $this->query($q);
if ($onlyone) {
return $this->fetch_assoc($r);
}
while ($l = $this->fetch_assoc($r)) {
$results[] = $l;
}
return $results;
}
public function get_error() {
return mysql_error();
}
public function close() {
if ($this->link_id) {
if ($this->query_result)
#mysql_free_result($this->query_result);
return #mysql_close($this->link_id);
}
else
return false;
}
public function auto_execute($table, $data, $type, $criteria='') {
$result = $this->get_select("desc " . $table);
if ($type == "INSERT")
$start = "insert into " . $table . " set ";
elseif ($type == "UPDATE")
$start = "update " . $table . " set ";
$sql = $start;
foreach ($result as $rst) {
foreach ($data as $key => $value) {
if ($key == $rst['Field'] and $key !== 0) {
if ((#ereg('date', $rst['Type'])) && $value == '') {
$sql = $sql . "`".$key."`" . "=NULL, ";
} elseif ((!#ereg('int', $rst['Type']))) {
$sql = $sql . "`".$key."`" . "='" . $value . "', ";
} else {
if (trim($value) != "") {
$sql = $sql . "`".$key."`" . "=" . $value . ", ";
}
}
}
}
}
if ($sql == $start)
return 0;
else {
$sql = substr($sql, 0, strlen($sql) - 2);
if ($type == "UPDATE" and !empty($criteria))
$sql = $sql . " where " . $criteria;
}
//echo $sql;exit;
if ($this->query($sql)) {
$return = $this->insert_id();
} else {
$return = 0;
}
return $return;
}
private function wplog($message) {
if(LOG==true){
$lineBreak = "\n"; // this function will NOT work on a windows server without further modification
$contents = date('Y-m-d H:i:s') . ' ' . $message. $lineBreak;
$myFile = SERVER_PATH.'/log.txt';
$fh = fopen($myFile, 'a') ;
fwrite($fh, $contents);
fclose($fh);
//SetFileContents(SERVER_PATH.'/log.txt',$contents,'a');
}
}
}