评论者认为您应该使用 mysqli 或 PDO 是正确的，但请尝试一下，看看它是否能解决您的问题:

The commenters are correct that you should be using mysqli or PDO, but try this out to see if it solves your issue:

if(isset($_POST)){ 
    $username = mysql_real_escape_string($_POST['user']); 
    $password = mysql_real_escape_string($_POST['pass']); 

    // lets check to see if the username already exists 

    $checkuser = mysql_query("SELECT username FROM register WHERE username='$username'");  

    $username_exist = mysql_num_rows($checkuser); 

    if($username_exist > 0){ 
        echo "I'm sorry but the username you specified has already been taken. Please pick another one."; 
        unset($username); 
        exit(); 
    }  

    // if no errors present with the username 
    // use a query to insert the data into the database. 

    $query = ("INSERT INTO register (username, password) VALUES ('$username', '$password')"); 
    mysql_query($query) or die(mysql_error()); 
    mysql_close(); 
} else {
    echo "No form submission detected.";
}

如果您仍有问题，我想查看您提交到此页面的表单以及用于注册的表格布局.

If you're still having issues, I'd like to see the form you're submitting to this page as well as the table layout for register.