Members system problem

[Anon]

Can anyone tell me why this code deosn't work? This file is login.php:
<?php if(!isset($submit)){
?>
<form method=get action="login.php">
Username:<input type="text" name="username"><br>
Password:<input type="password" name="password"><br>
<input type="submit" name="submit" value="Submit">
</form>
<?php
}
elseif(isset($submit)){
require('config.php');
mysql_connect($host, $user);
$query = mysql_db_query("members","SELECT * FROM details WHERE username=$username");
if($query["password"] == $password){
print("Access Granted<br><a href=file.php>Continue</a>");}
else{print("Either you are not signed up or you entered an incorrect password. Hit Back to try again.");}
}
?>

Whenever I try to use this system I get the "Either you are not signed up or..." message. Any ideas why?!

[Anon]

<?
if (!$submit) {
?>
<form method=post action="login.php">
Username:<input type="text" name="username"><br>
Password:<input type="password" name="password"><br>
<input type="submit" name="submit" value="Submit">
</form>

<?
}
if ($submit) {
include ('config.php');
$connection = mysql_connect ("$host", "$user", "$pass") or die ( "Couldn't connect to server.");
$db = mysql_select_db ("$dbase", $connection) or die ( "Couldn't select database.");
$sql = "select * from details where username=\"$username\" and password=\"$password\"";
$result = mysql_query($sql) or die ("Couldn't execute query.");
$check_num = mysql_num_rows($result);
if ($check_num == 0) {
echo "Either you are not signed up or you entered an incorrect password. Hit Back to try again.";
exit;
}
else {
echo "Access Granted<br><a href=file.php>Continue</a>";
}
}
?>

[owainhunt]

Cheers