用户名必须存在于数据库中以创建登录

Question

I created this login page with the help of Dreamweaver, and want to make it so the username field must match a username in the SQL database in order to create password. I am not very familiar with this and am unsure on what I need to do to require an existing username and then add a password to that username row in the database.

Here is the php code created by Dreamweaver: The connection to the database already exists, so I know that it is not present in the below code. This works just fine as a normal registration page, but I don't want just anybody being able to register.

<?php
if (!function_exists("GetSQLValueString")) {
function GetSQLValueString($theValue, $theType, $theDefinedValue = "", $theNotDefinedValue = "") 
{
  if (PHP_VERSION < 6) {
    $theValue = get_magic_quotes_gpc() ? stripslashes($theValue) : $theValue;
  }

  $theValue = function_exists("mysql_real_escape_string") ? mysql_real_escape_string($theValue) : mysql_escape_string($theValue);

  switch ($theType) {
    case "text":
      $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
      break;    
    case "long":
    case "int":
      $theValue = ($theValue != "") ? intval($theValue) : "NULL";
      break;
    case "double":
      $theValue = ($theValue != "") ? doubleval($theValue) : "NULL";
      break;
    case "date":
      $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
      break;
    case "defined":
      $theValue = ($theValue != "") ? $theDefinedValue : $theNotDefinedValue;
      break;
  }
  return $theValue;
}
}

// *** Redirect if username exists
$MM_flag="MM_insert";
if (isset($_POST[$MM_flag])) {
  $MM_dupKeyRedirect="userexists.php";
  $loginUsername = $_POST['username'];
  $LoginRS__query = sprintf("SELECT username FROM users WHERE username=%s", GetSQLValueString($loginUsername, "text"));
  mysql_select_db($database_User_Information, $User_Information);
  $LoginRS=mysql_query($LoginRS__query, $User_Information) or die(mysql_error());
  $loginFoundUser = mysql_num_rows($LoginRS);

  //if there is a row in the database, the username was found - can not add the requested username
  if($loginFoundUser){
    $MM_qsChar = "?";
    //append the username to the redirect page
    if (substr_count($MM_dupKeyRedirect,"?") >=1) $MM_qsChar = "&";
    $MM_dupKeyRedirect = $MM_dupKeyRedirect . $MM_qsChar ."requsername=".$loginUsername;
    header ("Location: $MM_dupKeyRedirect");
    exit;
  }
}

$editFormAction = $_SERVER['PHP_SELF'];
if (isset($_SERVER['QUERY_STRING'])) {
  $editFormAction .= "?" . htmlentities($_SERVER['QUERY_STRING']);
}

if ((isset($_POST["MM_insert"])) && ($_POST["MM_insert"] == "register")) {
  $insertSQL = sprintf("INSERT INTO users (username, password) VALUES (%s, %s)",
                       GetSQLValueString($_POST['username'], "text"),
                       GetSQLValueString($_POST['password1'], "text"));

  mysql_select_db($database_User_Information, $User_Information);
  $Result1 = mysql_query($insertSQL, $User_Information) or die(mysql_error());

  $insertGoTo = "login.php";
  if (isset($_SERVER['QUERY_STRING'])) {
    $insertGoTo .= (strpos($insertGoTo, '?')) ? "&" : "?";
    $insertGoTo .= $_SERVER['QUERY_STRING'];
  }
  header(sprintf("Location: %s", $insertGoTo));
}

mysql_select_db($database_User_Information, $User_Information);
$query_User_Request = "SELECT * FROM users";
$User_Request = mysql_query($query_User_Request, $User_Information) or die(mysql_error());
$row_User_Request = mysql_fetch_assoc($User_Request);
$totalRows_User_Request = mysql_num_rows($User_Request);
?>

Answer 1

Simply you may check this as

$loginUsername = $_POST['username'];
$LoginRS__query = sprintf("SELECT username FROM users WHERE username='$loginUsername'", GetSQLValueString($loginUsername, "text"));

if this query gives result successfully then after do you further logic