se vuoi una cosa semplice, questo potrebbe andarti bene:

<form name="control_log" method="post" action="control_log.php">
<table width="150" cellpadding="0" cellspacing="0" border="0" bordercolor="#666666" style="border-collapse:collapse">
<tr>
<td colspan="2" height="20" bgcolor="#D8CE52" align="center" valign="middle">
<font face="Verdana" style="font-size:11px">
Area Riservata
</font>
</td>
</tr>
<td colspan="2" height="1" bgcolor="#000000" valign="top"></td>
<tr>
<td width="85" bgcolor="#F9EC06" style="padding-left:5px; padding-top:3px">
<font face="Verdana" style="font-size:11px">
User ID
</font>
</td>
<td width="60" bgcolor="#F9EC06">
<input type="text" name="userid" id="userid" size="10" maxlength="10">
</td>
</tr>
<td width="85" bgcolor="#F9EC06" style="padding-left:5px; padding-top:3px">
<font face="Verdana" style="font-size:11px">
Password
</font>
</td>
<td width="60" bgcolor="#F9EC06">
<input type="password" name="password_user" id="password_user" size="10" maxlength="8">
</td>
</tr>
<tr>
<td colspan="2" align="right" bgcolor="#F9EC06">
<input type="submit" value="accedi">
</td>
</tr>
</table>
</form>


PAGINA control_log.php:

<?
require ("connessione.php");

$sql = "SELECT * FROM tabella where userid = '".$userid."' and password_user = '".$password_user."'" ;
$result = mysql_db_query($database,$sql,$db) or die("Errore ".mysql_error());
$num_rows = mysql_num_rows($result) ;
if ( $num_rows != 0 ) {
// trovato
$row = mysql_fetch_array( $result );
session_register( "loggato" );
$loggato = true;
header ("Location: pagina_riservata.php");
} else {
// non trovato
header ("Location: non_sei_autorizzato.php");
}
?>