有关php单例模式介绍及例子

private static $_instance;  

private function __construct()  
    {  
        $this->_db = pg_connect('xxxx'); 
    }  
    private function __clone() 
    { 
    }//覆盖__clone()方法，禁止克隆 

public static function getInstance()   
    {   
        if(! (self::$_instance instanceof self) )  
        {   
            self::$_instance = new self();   
        } 
        return self::$_instance;     
    }

<?php 
    ...... 
    //初始化一个数据库句柄 
    $db = new DB(...); 
     
    //添加用户信息 
    $db->addUserInfo(...); 
     
    ...... 
     
    //在函数中访问数据库，查找用户信息 
    function getUserInfo() 
    { 
        $db = new DB(...);//再次new 数据库类，和数据库建立连接 
        $db = query(....);//根据查询语句访问数据库 
    }
?>

<?php 
    class DB   
    {   
        private $_db;   
        private static $_instance;   
       
        private function __construct(...)   
        {   
            $this->_db = pg_connect(...);//postgrsql   
        }   
       
        private function __clone() {};  //覆盖__clone()方法，禁止克隆   
       
        public static function getInstance()   
        {   
            if(! (self::$_instance instanceof self) ) {   
                self::$_instance = new self();   
            }   
            return self::$_instance;   
        }
        public function addUserInfo(...) 
        {     
        } 
     
      public function getUserInfo(...) 
     {  
      }
    }
     
    //test     
    $db = DB::getInstance();     
    $db->addUserInfo(...);
    $db->getUserInfo(...); 
?>

<?php  
  /**
  数据库操作类
@link http://www.jb200.com
*/
    class db { 
        public $conn; 
        public static $sql; 
        public static $instance=null; 
        private function __construct(){ 
            require_once('db.config.php'); 
            $this->conn = mysql_connect($db['host'],$db['user'],$db['password']); 
            if(!mysql_select_db($db['database'],$this->conn)){ 
                echo "失败"; 
            }; 
            mysql_query('set names utf8',$this->conn);        
        } 
        public static function getInstance(){ 
            if(is_null(self::$instance)){ 
                self::$instance = new db; 
            } 
            return self::$instance; 
        } 
        /**
         * 查询数据库
         */ 
        public function select($table,$condition=array(),$field = array()){ 
            $where=''; 
            if(!emptyempty($condition)){ 
                 
                foreach($condition as $k=>$v){ 
                    $where.=$k."='".$v."' and "; 
                } 
                $where='where '.$where .'1=1'; 
            } 
            $fieldstr = ''; 
            if(!emptyempty($field)){ 
                 
                foreach($field as $k=>$v){ 
                    $fieldstr.= $v.','; 
                } 
                 $fieldstr = rtrim($fieldstr,','); 
            }else{ 
                $fieldstr = '*'; 
            } 
            self::$sql = "select {$fieldstr} from {$table} {$where}"; 
            $result=mysql_query(self::$sql,$this->conn); 
            $resuleRow = array(); 
            $i = 0; 
            while($row=mysql_fetch_assoc($result)){ 
                foreach($row as $k=>$v){ 
                    $resuleRow[$i][$k] = $v; 
                } 
                $i++; 
            } 
            return $resuleRow; 
        } 
        /**
         * 添加一条记录
         */ 
         public function insert($table,$data){ 
            $values = ''; 
            $datas = ''; 
            foreach($data as $k=>$v){ 
                $values.=$k.','; 
                $datas.="'$v'".','; 
            } 
            $values = rtrim($values,','); 
            $datas   = rtrim($datas,','); 
            self::$sql = "INSERT INTO  {$table} ({$values}) VALUES ({$datas})"; 
            if(mysql_query(self::$sql)){ 
                return mysql_insert_id(); 
            }else{ 
                return false; 
            }; 
         } 
         /**
          * 修改一条记录
          */ 
        public function update($table,$data,$condition=array()){ 
            $where=''; 
            if(!emptyempty($condition)){ 
                 
                foreach($condition as $k=>$v){ 
                    $where.=$k."='".$v."' and "; 
                } 
                $where='where '.$where .'1=1'; 
            } 
            $updatastr = ''; 
            if(!emptyempty($data)){ 
                foreach($data as $k=>$v){ 
                    $updatastr.= $k."='".$v."',"; 
                } 
                $updatastr = 'set '.rtrim($updatastr,','); 
            } 
            self::$sql = "update {$table} {$updatastr} {$where}"; 
            return mysql_query(self::$sql); 
        } 
        /**
         * 删除记录
         */ 
         public function delete($table,$condition){ 
            $where=''; 
            if(!emptyempty($condition)){ 
                 
                foreach($condition as $k=>$v){ 
                    $where.=$k."='".$v."' and "; 
                } 
                $where='where '.$where .'1=1'; 
            } 
            self::$sql = "delete from {$table} {$where}"; 
            return mysql_query(self::$sql); 
             
         } 
         
        public static function getLastSql(){ 
            echo self::$sql; 
        }        
    } 
     
    $db = db::getInstance(); 
    //$list = $db->select('demo',array('name'=>'tom','password'=>'ds'),array('name','password')); 
    //echo $db->insert('demo',array('name'=>'最近你啦','password'=>'123')); 
    //echo $db->update('demo',array("name"=>'xxx',"password"=>'123'),array('id'=>1)); 
    echo $db->delete('demo',array('id'=>'2')); 
    db::getLastSql(); 
    echo "<pre>"; 
?>