php单例模式

<?php
/**
* $_instance必须声明为静态的私有变量
* 构造函数和析构函数必须声明为私有,防止外部程序new
* 类从而失去单例模式的意义
* getInstance()方法必须设置为公有的,必须调用此方法
* 以返回实例的一个引用
* ::操作符只能访问静态变量和静态函数
* new对象都会消耗内存
* 使用场景:最常用的地方是数据库连接。 

* 使用单例模式生成一个对象后，
* 该对象可以被其它众多对象所使用。 
**/
$db = array(  
        ‘host‘=>‘localhost‘,  
        ‘user‘=>‘root‘,  
        ‘password‘=>‘‘,  
        ‘database‘=>‘test‘,  
);  
class db {  
    private $conn;  
    public static $sql;  
    private static $instance;  
    private function __construct(){  
        $this->conn = mysql_connect($db[‘host‘],$db[‘user‘],$db[‘password‘]);  
        if(!mysql_select_db($db[‘database‘],$this->conn)){  
            echo "失败";  
        };  
        mysql_query(‘set names utf8‘,$this->conn);         
    }  
     //创建__clone方法防止对象被复制克隆
    private function __clone(){
        //trigger_error(‘Clone is not allow!‘,E_USER_ERROR);
    }
    //单例方法,用于访问实例的公共的静态方法
    public static function getInstance(){  
        if(is_null(self::$instance)){  // !(self::$_instance instanceof self)
            self::$instance = new db;  
        }  
        return self::$instance;  
    }  
     /** 
     * 查询数据库 
     */  
    public function select($table,$condition=array(),$field = array()){  
        $where=‘‘;  
        if(!empty($condition)){   
            foreach($condition as $k=>$v){  
                $where.=$k."=‘".$v."‘ and ";  
            }  
            $where=‘where ‘.$where .‘1=1‘;  
        }  
        $fieldstr = ‘‘;  
        if(!empty($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()){  
        $where=‘‘;  
        if(!empty($condition)){  
            foreach($condition as $k=>$v){  
                $where.=$k."=‘".$v."‘ and ";  
            }  
            $where=‘where ‘.$where .‘1=1‘;  
        }  
        $updatastr = ‘‘;  
        if(!empty($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){  
        $where=‘‘;  
        if(!empty($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();  
echo $db->delete(‘demo‘,array(‘id‘=>‘2‘));  
db::getLastSql();