Ti ho preparato un form per login molto rudimentale giusto per farti capire il funzionamento:

Codice PHP:


<!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=iso-8859-1" />
<title>Documento senza titolo</title>
<style type="text/css">
<!--
.login {
    font-size: 36px;
    font-family: Tahoma;
    color: #990000;
    font-weight: bold;
}
.textbox {
    border-bottom-color:#CCCCCC;
    border-bottom-style:solid;
    border-bottom-width:1px;
    border-top-color:#CCCCCC;
    border-top-style:solid;
    border-top-width:1px;
    border-left-color:#CCCCCC;
    border-left-style:solid;
    border-left-width:1px;
    border-right-color:#CCCCCC;
    border-right-style:solid;
    border-right-width:1px;
    font-family: arial;
    font-size: 11px;
}
.testo {
    font-family: arial;
    font-size: 11px;
}
-->
</style>
</head>

<body>
<div align="center">
  <table width="329" border="0" cellpadding="0" cellspacing="0" class="textbox">
    
    <tr>
      <td height="50" colspan="3" valign="top"><div align="center" class="login">LOGIN</div></td>
    </tr>
    <tr>
      <td width="100" height="40"></td>
      <td width="26"></td>
      <td width="201"></td>
    </tr>
    <tr>
      <td height="19" valign="top" class="testo"><div align="right">USER:</div></td>
      <td></td>
      <td rowspan="3" valign="top"><form id="form1" name="form1" method="post" action="login.php">
        <label>
          <input type="text" name="username" class="textbox"/>
          </label>

        <label>
          <input type="password" name="password" class="textbox"/>
          </label>


          <input name="submit" type="submit" value="Login"/>
      </form></td>
    </tr>
    <tr>
      <td height="19" valign="top" class="testo"><div align="right">PASS:</div></td>
    <td></td>
    </tr>
    <tr>
      <td height="73"></td>
      <td></td>
    </tr>
    <tr>
      <td height="41"></td>
      <td></td>
      <td></td>
    </tr>
  </table>
</div>
</body>
</html> 
Codice PHP:
<?php
    // QUESTA E' LA PAGINA PHP (login.php)
    // ACQUISISCO LE VARIABILI INVIATE DAL FORM
    $user = $_POST['username'];
    $passwd = $_POST['password'];
    
    // CONTROLLO I DATI DEL LOGIN
    if (($user == "amministratore") && ($passwd == "123456")){
        echo "LOGIN EFFETTUATO CON SUCCESSO!";
    } else {
        echo "LOGIN ERRATO";
    }
?>
Ovviamente la gestione degli utenti è una procedura un po più delicata... ma, come ben sai, le case si costruiscono mattone dopo mattone...