Warning: Cannot modify header information - headers already sent by (output sta

This code below gives me this error message: Warning: Cannot modify header information - headers already sent by (output started at /var/www/html/account.php:6) in /var/www/html/account.php on line 17 The script check in the database if user exists and than redirect the page to the home index.php page. I know the problem exist in the header() .. But I don't understand why? <?php session_start(); include_once("lib/lib_main.php"); dbConnect($host, $user, $pass); echo "<FORM ACTION=\"$PHP_SELF\" METHOD=\"POST\">";?> <table align="center"> <tr><td>Username:</td><td> <input type="username" name="username"></td></tr> <tr><td>Password:</td><td> <input type="password" name="password"></td></tr> <tr><td></td><td align="center"><input type="submit" name="submit" value="Submit"></td><tr> <?php echo "</FORM>"; $password = $_POST['password']; $username = $_POST['username']; $sql_login = mysql_query("SELECT id from user WHERE username='$username' and password=password('$password')"); if(mysql_num_rows($sql_login)) { header("Location: http://www.index.php"); } ?> _________________________________________________________________ Find things fast with the new MSN Toolbar – includes FREE pop-up blocking! http://clk.atdmt.com/AVE/go/onm00200414ave/direct/01/