post取不到值

form:
<form name=form1 action="admin.php" method="post">
<p><label>管理员账号：</label><input name="adminname" type="text" /></p>
<p><label>密码：</label><input name="adminpwd" type="password" /></p>
<p><label>验证码：</label><img src="checkno.php" alt="验证码" onclick="this.src=this.src"/><input name="checkno" type="text"  class="checkno"/></p>
<p class="bt"><input name="LogBtn" type="submit"  value="登录"/></p>
<input name="act" type="hidden"  value="checkadmin"/>
</form>checkadmin:
if($_REQUEST['act']=='checkadmin'){
$aname=$_POST['adminname'];
$apwd=$_POST['adminpwd'];
$checkno=$_POST['checkno']; if($checkno!=$_SESSION['randcode']){
AlertBack(("验证码不对！"));
}
$admin=new MyData();
$admininfo=$admin->getLogin('admin','aname',$aname,'apwd',$apwd);
if($admininfo){
setcookie("ckaid",$admininfo['aid']);
setcookie("ckaname",$admininfo['aname']);
header("location:index.php");
}
else{
AlertBack("账号或密码不对！");
}