PHP mysqli database operation class

<?php
class Mysql{
	private $host = &#39;localhost&#39;;
	private $port = &#39;3306&#39;;
	private $user = &#39;username&#39;;
	private $pwd = &#39;password&#39;;
	private $db = &#39;dbname&#39;;
	private $char = &#39;UTF8&#39;;
	private $prefix = &#39;&#39;;
	private $fetch_mode = MYSQLI_ASSOC;//获取模式
	private $result;//结果集
	public $mysqli;//mysqli实例<strong>对象</strong>
	static private $_instance;//本类实例
	
	//构造函数初始化$mysqli<strong>对象</strong>
	private function __construct() {
		$this->mysqli=new mysqli($this->host,$this->user,$this->pwd,$this->db,$this->port);
		if(mysqli_connect_errno()){
			$this->mysqli=false;
			echo mysqli_connect_error();
			die();
		}else{
			$this->mysqli->set_charset($char);	
		}
	}
	
	//析构函数：释放结果集和关闭数据库
	public function __destruct(){
		$this->free();
		$this->close();	
	}
	
	//初始化$mysqli<strong>对象</strong>
	public static function getInstance(){
		if(!(self::$_instance instanceof self)) {
			self::$_instance = new self();
		}
		return self::$_instance;
	}
	
	//释放结果集
	private function free(){
		@$this->result->free();
	}
	
	//关闭数据库连接
	private function close(){
		$this->mysqli->close();	
	}
	
	//执行sql语句
	public function query($sql){
		return $this->mysqli->query($sql);	
	}
	
	//获取查询结果
	public function get_result_array($table,$field,$c
		$table=$this->prefix.$table;
		if(is_array($field)){
			$field = join(',',$field);
		}
		
		$sql = "SELECT $field FROM ".$table;
		if(!empty($condition))$sql .=" $condition ";
		$this->result = $this->query($sql);
		$return = array();
		while($row = $this->fetch($this->result)){
			$return[] = $row;
		}
		$this->free();
		return $return;
	}
	
	//增删改操作
	public function execute($table,$action,$arr_field=array(),$c
		$table=$this->prefix.$table;
		switch($action){
			case 'INSERT':
				$str_field = '';
				$str_val = '';
				foreach($arr_field as $key=>$val){
					$str_field .= '`'.$key.'`,';
					$str_val .= '\''.$val.'\',';
				}
				$str_field = rtrim($str_field, ',');
				$str_val = rtrim($str_val, ',');
				$sql = "INSERT INTO $table ($str_field) VALUES ($str_val) ";
			break;
			
			case 'DELETE':
				$sql = "DELETE FROM $table";
				if (!empty($condition)) $sql .= " WHERE $condition";
			break;
			
			case 'UPDATE':
				$str_field = '';
				foreach($arr_field as $key => $val){
					$str_field.= '`'.$key ."` ='$val',";
				}
				$str_field = rtrim($str_field, ',');
				$sql = "UPDATE $table SET $str_field";
				if (!empty($condition)) $sql .= " WHERE $condition";
			break;	
		}
		$this->query($sql);
		return $this->get_affected_rows();
	}
	
	//获得受影响行数(针对增删改操作)
	public function get_affected_rows(){
		return $this->mysqli->affected_rows;
	}
	
	//获取集合条数
	public function get_rows($table,$c 1 ',$id='id'){
		$table=$this->prefix.$table;
		$sql="SELECT COUNT($id) num FROM ".$table." $condition";
		$this->result=$this->query($sql);
		$row=$this->fetch($this->result);
		return $row['num'];
	}
	
	//获得结果集
	public function fetch($result){
		return $result->fetch_array($this->fetch_mode);
	}
	
	//获得所有结果集
	public function fetch_all($result){
		$rows=array();
		while($row=$this->fetch($result)){
			$rows[]=$row;	
		}
		return $rows;
	}
}

I want to use PHP to write a database operation class encapsulated by the interface of mobile App

The above introduces the PHP mysqli database operation class, including object content. I hope it will be helpful to friends who are interested in PHP tutorials.