查询构造器基类

<?php
namespace phpedu;
use PDO;
class DB{
    protected $db;
    protected $table;
    protected $field;
    protected $limit;
    protected $opt = [];
    public function  __construct($dsn,$username,$password){
        $this->db = new PDO($dsn,$username,$password);
    }
    // 获取数据表
    public function table($table){
        $this->table = $table;
        return $this;
    }
    // 获取查询字段
    public function field($field){
        $this->field = $field;
        return $this;
    }
    // 获取位置限制条件
    public function where($where=''){
        $this->opt["where"] = " where $where";
        return $this;
    }
    // 获取查询限制
    public function limit($limit=10){
        $this->limit = $limit;
        $this->opt["limit"] = " LIMIT ".$this->limit;
        return $this;
    }
    // 获取偏移量
    public function page($page=1){
        $this->opt["offset"] =  " OFFSET ".($page-1)*$this->limit;
        return $this;
    }
    // 清理查询条件助手
    private function cleaner(){
        foreach($this->opt as $key=>$item){
            $this->opt[$key]=null;
        }
    }
    private function execute($sql){
        $stmt = $this->db->prepare($sql);
        $stmt->execute();
        $this->cleaner();
        return $stmt;
    }
    // 查
    public function select(){
        $sql = 'SELECT ' . $this->field . ' FROM ' . $this->table;
        $sql .= $this->opt["where"] ?? null;
        $sql .= $this->opt["limit"] ?? null;
        $sql .= $this->opt["offset"] ?? null;
        return $this->execute($sql)->fetchAll();
    }
    // 增
    public function insert($data){
        $str = '';
        foreach ($data as $key=>$value){
            $str .= $key.'="'. $value .'", ';
        }
        $sql = 'INSERT '.$this->table . ' SET ' . rtrim($str, ', ');
        return $this->execute($sql)->rowCount();
    }
    // 改
    public function update($data){
        $str = '';
        foreach ($data as $key=>$value){
            $str .= $key.'="'. $value .'", ';
        }
        $sql = 'UPDATE '.$this->table . ' SET ' . rtrim($str, ', ');
        $sql .= $this->opt['where'] ?? die('禁止无条件更新');
        return $this->execute($sql)->rowCount();
    }
    // 删
    public function delete(){
        $sql = 'DELETE FROM ' . $this->table;
        $sql .= $this->opt['where'] ?? die('禁止无条件删除');
        return $this->execute($sql)->rowCount();
    }
}
$db = new Db('mysql:dbname=phpedu','root','root');
$db_select = $db->table('staff')->field('id,name,gender')->limit(2)->select();
printf("<pre>%s</pre>",print_r($db_select,true));
$db->insert(['name'=>'David', 'gender'=>0, 'email'=>'David@qq.com']);
$db->where('id = 1')->update(['name'=>'David', 'gender'=>0]);
$db_select = $db->table('staff')->field('id,name,gender')->limit(1)->select();
printf("<pre>%s</pre>",print_r($db_select,true));
$db->where('id = 1')->delete();
$db_select = $db->table('staff')->field('id,name,gender')->limit(1)->select();
printf("<pre>%s</pre>",print_r($db_select,true));