我正在使用登录/注册系统，但我遇到了这个错误，我似乎无法弄明白。每当我尝试注册时，无论我输入什么值，页面都会刷新，然后该页面的所有CSS和HTML都会消失。这是代码：
if ($_POST['register']) {
if ($_POST['username'] && $_POST['password'] && $_POST['passwordconfirm'] && $_POST['fullname']) {
$username = mysql_real_escape_string($_POST['username']);
$password = mysql_real_escape_string(hash("sha512", $_POST['password']));
$passwordconfirm = mysql_real_escape_string(hash("sha512", $_POST['password']));
$fullname = my_real_escape_string($_POST['fullname']);
if ($_POST['fullname']) {
$name = mysql_real_escape_string(strip_tags($_POST['name']));
};
$check = mysql_fetch_array(mysql_query("SELECT * FROM 'users' WHERE 'Username='$username'"));
if ($check != '0') {
die("That username already exists! Click <a href='http://growtapians.com/Login & Register System/register.php'>here</a> and try entering a different one.");
};
if(!ctype_alnum($username)) {
die("Sorry, but only numbers and letters are allowed in the username. Click <a href='http://growtapians.com/Login & Register System/register.php'>here</a> and try removing all special characters!");
};
if (strlen($username) > 20) {
die("The username cannot be more than 20 characters long. Click <a href='http://growtapians.com/Login & Register System/register.php'>here</a> and try shortening your username!");
};
if ($password != $passwordconfirm) {
die("The passwords did not match! Click <a href='http://growtapians.com/Login & Register System/register.php'>here</a> and try again!");
};
$salt = hash("sha512", rand() . rand() . rand());
mysql_query("INSERT INTO 'users' ('Username', 'Password', 'Name', 'Salt') VALUES ('$username', '$password', '$fullname', '$salt')");
setcookie("c_user", hash("sha412", $username), time() + 24 * 60 * 60, "/");
setcookie("c_salt", $salt, time() +24 * 60 * 60, "/");
die("Registration successful!");
};
};
如果需要，这是HTML和CSS：
<!DOCTYPE html>
<html lang='en'>
<head>
<meta charset='UTF-8'>
<meta name='viewport' content='width=device, height=device=height, initial-scale=1'>
<style media='screen'>
body {
font: 1em/1.62em verdana, sans-serif;
background-color: #249EC7;
}
form {
max-width: 58em;
padding: .2em;
margin: auto;
background-color: #648cd1;
color: #31d8eb;
text-align: center;
}
form div {
margin: 0em 1em 1em 1em;
}
form b {
font-size: 1.3em;
}
form b,form input{
display: inline-block;
width: 12em;
}
form input {
padding: 0.25em;
}
h4 a:visited {
color: #F56433;
}
#logindiv {
height: 18.5em;
margin-top: -9.25em;
</style>
<body>
<div id='logindiv' style='width: 50%; padding: 10px; border: 5px solid #316ED6; background-color: #648CD1; color: #31D8EB; margin: auto; text-align: center;'>
<h1>Login</h1>
<br />
<form action='' method='post'>
<div>
<b>Username:</b>
<input type='text' name='username' style='padding: 4px;'/>
</div>
<div>
<b>Password:</b>
<input type='password' name='password' style='padding: 4px; '/>
</div>
<div>
<input type='submit' value='Login' name='login'/>
</div>
</form>
<div>
<h4>No Account? Register <a href='register.php'>Here!</a></h4>
</div>
</div>
</body>