JS表单里怎样写唯一标示符呢？

应该在php生成唯一标准，放入你的表单中

 <?php
$_SESSION['st']=$st=sha1(uniqid[rand(),true));
?>
layer += '<div class="layer_title"><h1>注册</h1><a href="javascript:;" title="关闭">x</a></div>'; 
        layer += '<form action="register.php" method="POST" id="registerForm">';
                layer += '<input type="hidden" name="st" value="<?php echo $st;?>"/>'; //添加隐藏控件存储唯一id
        layer += '<ul>'; //ul放下来
         layer += '<b>*</b>：</span><input type="hidden"  name="uniqid" id="uniqid" value="<?php $st?>"/>'; 
        layer += '<li><span>账　　号<b>*</b>：</span><input type="text" class="layer_txt" placeholder="至少两位" name="username" id="uname" /></li>'; 
        layer += '<li><span>密　　码<b>*</b>：</span><input type="password" class="layer_txt" placeholder="至少六位" name="password" id="pwd" /></li>'; 
        layer += '<li><span>确认密码<b>*</b>：</span><input type="password" class="layer_txt" placeholder="至少六位" name="notpassword" id="repwd" /></li>';
        layer += '<li><span>电子邮箱<b>*</b>：</span><input type="text" class="layer_txt" placeholder="您常用的邮箱地址" name="email" id="email" /></li>';            
        layer += '</form></ul>';
        layer += '<div class="layer_btn"><input type="button" id="ressub" class="layer_submit_btn" value="提交" /></div>'; 

 $st=$_POST["st"];