Problem: when validation in username and password in php

I got error in username during validation by this code. Anything this code, Please fix it? I am newbie for php.

 <?php
    session_start(); 
           include('functions.php');

if($_SERVER['REQUEST_METHOD']=="POST"){
       $qer="select * from admin where username='".$_POST['username']."' and  password='".$_POST['password']."'";
  $res=mysql_query(qer);
  $num=mysql_num_rows($res);
 if($num==0)

 {
$msg=1;
 }
else if($num==1)
{
session_unregister("user_name");
 session_register("user_name");
 $_SESSION["user_name"]=_POST["username"];

session_unregister("adminid");
session_register("adminid");
$_SESSION['adminid']=getdata("admin","id","username='"$_POST['username']."' and     password='"$_POST['password']."'");

  echo'<script language="ja开发者_如何学编程vascript">window.location.href="welcome.php";</script>';
  }
   } ?>

You should always put the error along with your question and code.

$res=mysql_query(qer);
  $num=mysql_num_rows($res);
 if($num==0)

Should be

$res=mysql_query($qer);
  $num=mysql_num_rows($res);
 if($num==0)

You forgot a $