php - Username MUST exist in database to create login -

i created login page of dreamweaver, , want make username field must match username in sql database in order create password. not familiar , unsure on need require existing username , add password username row in database.

here php code created dreamweaver: connection database exists, know not present in below code. works fine normal registration page, don't want being able 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'];   $loginrs__query = sprintf("select username users username=%s", getsqlvaluestring($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 row in database, username found - can not add requested username   if($loginfounduser){     $mm_qschar = "?";     //append username redirect page     if (substr_count($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")) {   $insertsql = sprintf("insert users (username, password) values (%s, %s)",                        getsqlvaluestring($_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 * 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); ?> 

simply may check

$loginusername = $_post['username']; $loginrs__query = sprintf("select username users username='$loginusername'", getsqlvaluestring($loginusername, "text")); 

if query gives result after further logic