I have not seen your form code posted yet, so it is also likely that the form is invalid and is not even submitting any $_POST data.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Son of Mini-Missions - Home</title>
<style type="text/css">
<!--
body {
background-image: url(http://www.jservers.co.uk/imgs/bg.png);
background-repeat: repeat;
}
#apDiv1 {
position:absolute;
left:541px;
top:296px;
width:266px;
height:26px;
z-index:1;
font-size: 16px;
font-family: Georgia, "Times New Roman", Times, serif;
font-weight: bold;
}
.loginHolder tr td b {
font-weight: bold;
}
-->
</style></head>
<body>
<div align="center">
<p><a href="index.php"><img src="logo.jpg" width="648" height="144" /></a></p>
<table class="loginHolder">
<tr>
<td><b>Player Name</b></td>
<td><b>Password:</b></td>
</tr>
<tr>
<td height="24"><input name="username" type="text" value="My_SonofMM_PlayerName" /></td>
<td><input name="password" type="password" id="loginPassword" value="MyPassword" /></td>
</tr>
</table>
<form action="login.php" method="post">
<input type="submit" name="button" id="button" value="Login" />
</form>
<hr />
<p align="center"> </p>
</div>
</body>
</html>