Please help php coding

<?php
session_start();
if(isset($_SESSION["manager"]))
{
header("location:index.php");
exit();
}

?>
<?php
if(isset($_POST["username"])&& isset($_POST["password"]));
{
$manager = preg_replace('#[^A-Za-z0-9]#i','',$_POST["username"]);
$password = preg_replace('#[^A-Za-z0-9]#i','',$_POST["password"]);
include"../storescripts/connect_to_mysql.php";
$sql = mysql_query("SELECT id FROM admin WHERE
username='$manager' AND
password='$password' LIMIT 1");

$existCount = mysql_num_rows($sql);
if ($existCount == 1)
{
while ($row = mysql_fetch_array($sql)){
$id = $row["id"];
}
$_SESSION["id"] = $id;
$_SESSION["manager"] = $manager;
$_SESSION["password"] = $password;
header("location:index.php");
exit();
}
else {echo 'That Information is incorrect, try again
<a href="index.php">Click Here</a>';
exit();
}
}


its ment to be a login to a inventory management section of a website but it keeps skipping past the html form and straight to the echo that is in the last part of the script. Any help would be amazing.