Singleton Design Pattern Example in PHP

Here is a sample example class that implements Singleton Design Pattern in PHP:

require_once("properties.php");
class DbConnection
{
 private $connectionId;
 
 private function __construct($dbHost,$dbUser,$dbPassword,$dbName)
 {
  $this->connectionId = 
  mysql_connect($dbHost,$dbUser,$dbPassword,true);
  if($this->connectionId)
  {
   $this->selectDatabase($dbName);
  }
  else
  {
   throw new Exception(mysql_error());
  }
 }
 
 private function selectDatabase($db)
 {
  $dbSelected = mysql_select_db($db,$this->connectionId);
  if(!$dbSelected)
  {
   throw new Exception(mysql_error());
  }
 }
 
 function __destruct()
 {
  if(is_resource($this->connectionId))
  {
   $this->closeConnection();
  }
 }
 
 public function closeConnection()
 {
  mysql_close($this->connectionId);
 }
 
 public function getResourceId()
 {
  return $this->connectionId;
 }
 
 public static function getInstance()
 {
  static $dbConnObj;
  if(!isset($dbConnObj))
  {
   $dbConnObj = new DbConnection(HOSTNAME,DB_USER,DB_PWD,DB_NAME);
  }
  return $dbConnObj;
 }
 
 public function prepareQuery($args=null)
 {
  $resourceId = $this->getResourceId();
  $args = func_get_args();
  $query = array_shift($args);
  foreach($args as $argIndex=>$param)
  {
   $args[$argIndex] = mysql_real_escape_string($param,$resourceId);
  }
  return vsprintf($query,$args);
 }
 
 public function fetchRow($query)
 {
  $recordDetails = $this->executeQuery($query);
  return $recordDetails[0]? get_object_vars($recordDetails[0]) : null;
 }
 
 public function fetchVar($query)
 {
  $recordDetails = $this->fetchRow($query);
  if(count($recordDetails)>0)
  {
   $values = array_values($recordDetails);
  }
   
  if(isset($values[0]) && $values[0]!=='')
  {
   return $values[0];
  }
 }
 
 public function executeQuery($query)
 {
  $resourceId = $this->getResourceId();
  $result = @mysql_query($query,$resourceId);
  if( preg_match("/^\\s*(insert|update|delete|replace|alter)/i",$query) ){
   $returnVal = mysql_affected_rows($resourceId); 
  }
  else{
   $resultSet = array();
   while($row = @mysql_fetch_object($result))
   {
    $resultSet[] = $row; 
   }
   $returnVal = $resultSet;
  }
  return $returnVal;
 }
}

try{
 $dbConn = DbConnection::getInstance(); 
}catch(Exception $e){
 die("
Error:".$e->getMessage());
}

Note:
1. HOSTNAME
2. DB_USER
3. DB_PWD
4. DB_NAME are constants defined in

properties.php