试写了一个登陆界面，还没有完成，测试的时候发现问题。$_POST里的值不会清空，每输入一次用户名，$_POST里面就多一个用户名，后果就是第一次错误输入的话，就无法再正确输入用户名。。。除非浏览器刷新页面。。。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta content="en-us" http-equiv="Content-Language" /><meta content="text/html; charset=utf-8" http-equiv="Content-Type" /><title>Welcome back</title><style type="text/css">.auto-style2 { text-align: center;}.auto-style1 { font-size: 36pt; text-align: left;}.auto-style4 { font-family: Broadway; font-size: xx-large;}</style></head><body><table align="center" style="width: 100%"> <tr> <td> <div class="auto-style2"> <table align="center" style="width: 100%"> <tr> <td class="auto-style1">Welcome back</td> <td> <img style="max-width:90%" src="logo.jpg" style="float: right" style="max-width:90%" / alt="该如何清空$_POST里的数据？" ></td> </tr> </table> </div> <hr /> <p class="auto-style4">Log in:</p> <form action="login.php" method="post" style="height: 213px"> <div class="auto-style2"> <table style="width: 100%"> <tr> <td class="auto-style2">User Name:</td> </tr> <tr> <td class="auto-style2"> <input name="user_name" type="text" value=" <?php if(isset($_POST['user_name'])) { print htmlspecialchars($_POST['user_name']); } ?> "/> </td> </tr> <tr> <td class="auto-style2">Password:</td> </tr> <tr> <td class="auto-style2"> <input name="password" type="password" /></td> </tr> </table> <br /> <br /> <input name="submit" type="submit" value="Sign in" /></div> </form> </td> </tr></table><?phprequire_once 'connectDB.php';print '<style type = "text/css" media = "screen">.error {color: red;}</style>';if($_SERVER['REQUEST_METHOD']=='POST'){ $problem = false; $login = false; if(empty($_POST['user_name'])) { $problem = true; print '<p class = "error">Please enter your user name!</p>'; } if(empty($_POST['password'])) { $problem = true; print '<p class = "error">Please enter a password</p>'; } if(!$problem) { $user = trim(stripslashes($_POST['user_name'])); $pwd = trim(stripslashes($_POST['password'])); $user = mysql_real_escape_string($user); $pwd = mysql_real_escape_string($pwd); $q = "SELECT * FROM USER WHERE USER_NAME = '$user' and PASSWORD = '$pwd'"; echo $q; $result = mysql_query($q); if(!$result || mysql_num_rows($result) == 0) { print '<p class = "error">Wrong Username or Password.</p>'; } else { $row = mysql_fetch_array($result); $id = $row['USER_ID']; $name = $row['USER_NAME']; echo "User ID is ".$id; session_start(); $_SESSION['$id'] = $id; //clear the POST array $_POST = array(); //网上找来的办法， 但貌似不管用 } } else { print '<p class = "error">Please try again!</p>'; } }?></body></html>
<input name="user_name" type="text" value=" <?php if(isset($_POST['user_name'])) { print htmlspecialchars($_POST['user_name']); } ?> "/> <input name="user_name" type="text" value="<?php if(isset($_POST['user_name'])) { print htmlspecialchars($_POST['user_name']); } ?>"/>PHP code?12345678 谢谢版主大神。小弟刚刚学习php和网页制作，大神是否能解释一下，空格和回车符作怪的原因吗？
input type=text 是单行文本编辑器
如果你给他的是多行的文本，那么只有第一行可以被编辑。这就是你遇到的问题的原因
怎么会有这种事？？？
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我是来学习的
可能是 你post 赋值时有问题,post 的值只会覆盖上一个,怎么会附加呢..
