PHP simple login check class

<?php
class checklogin
{
  var $name;
  var $pwd;
  function __construct($username,$password)
  {
   $this->name=$username;
   $this->pwd=$password;
  }
  function checkinput()
  {
   global $db;
   $sql="select * from tb_manager where name=&#39;$this->name&#39; and pwd=&#39;$this->pwd&#39;";
   $res=$db->query($sql);
   $info=$db->fetch_array($res); 
   if($info[&#39;name&#39;]==$this->name and $info[&#39;pwd&#39;]==$this->pwd)
   {
     $_SESSION[admin_name]=$info[name];
     $_SESSION[pwd]=$info[pwd];
     echo "<script>alert(&#39;登录成功！);window.location.href=&#39;index.php&#39;;</script>";
   }
   else
   {
     echo "<script language=&#39;javascript&#39;>alert(&#39;登录失败！&#39;);history.back();</script>";
     exit;
   }
  }
}
?>