数据库请求委托代理

数据库请求委托代理

作业标题：0816 oop编程-4
作业内容：请用请求委托原理封装查询构造器（只封装查询操作即可）

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• 请求委托原理封装查询构造器

class Query{    
private static $db;
protected $table;
protected $field;
protected $limit;   
private function __construct()
{   
}  
static function connect($dsn,$username,$pwd)
{
    //创建PDO类的唯一实例 pdo对象
    if(is_null(static::$db))
    {
        static::$db = new PDO($dsn,$username,$pwd);
    }
    // 返回query实例
    return new static();
}
public function table($table)
{
    $this->table = $table;
    return $this;
}
public function field($field)
{
    $this->field = $field;
    return $this;
}
public function limit($limit)
{
    $this->limit = $limit;
    return $this;
}
public function getSql()
{
    return sprintf('SELECT %s FROM %s LIMIT %d ',$this->field,$this->table,$this->limit);
}
public function select()
{
    return static::$db->query($this->getSql())->fetchAll(PDO::FETCH_ASSOC);
}
}
class Db{
static function __callStatic($method,$args)
{
    $dsn = 'mysql:host=localhost;dbname=my_test';
    $username = 'root';
    $pwd = 'root';
    // 获取到被委托的类query实例
    $query = Query::connect($dsn,$username,$pwd);
    return call_user_func([$query,$method],...$args);
} 
}$res = Db::table('emp')->field('ename,empno')->limit(5)->select();
echo '<pre>';
print_r($res);