PHP基于单例模式实现的数据库操作基类

<?php
$db = array(
    'host'=>'localhost',
    'user'=>'root',
    'password'=>'',
    'database'=>'test',
)
?>

<?php
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()){
    $where='';
    if(!empty($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;
  }
}
$db = db::getinstance();
//$list = $db->select('demo',array('name'=>'tom','password'=>'ds'),array('name','password'));
//echo $db->insert('demo',array('name'=>'','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>";
?>