PHp - submit page returns Internal Server Error

Question 1

The "Internal Server Error" could mean several things, but most likely means an error in your PHP code. This will require setting the display_errors property to "1" in your php.ini. The problem might also be a startup error, so you may wish to also consider the display_startup_errors property.

http://www.php.net/manual/en/errorfunc.configuration.php#ini.display-errors

Two comments about the code, unrelated to your question: 1) the expression $ipass !== $ipass seems like a typo, as it will always return FALSE, and 2) this "security" is easily bypassed by turning javascript off. Consider using header() for redirects instead.

http://us2.php.net/manual/en/function.header.php

EDIT: ... in this specific case, the error is use of the variable $iuser, which is undefined. You'd previously declared it as $iusr. Turning on error reporting or looking at the log will present you with good error messages to find these sorts of problems easily.

Question 2

login.php

<html>
<link rel="stylesheet" type="text/css" href="style.css">
<body>
    <center><img src="logo.png"></center>
    <div id="login">
        <form action="protected.php" method="POST">
        </br>
        Username  <input type="text" name="user" class="text"/>
        <p></p>
        Password <input type="password"" name="password" class="text" />
        <p></p>
            <input type="submit" name="submit" value="Login" class="button">
        </form>
    </div>
</body>

protected.php

<?php
$usr = "admin";
$pass = "admin";

$iusr = $_POST["user"];
$ipass = $_POST["password"];
if(isset($_POST["submit"])){
if ($iusr != $usr || $ipass != $pass) {
?>
<html>
<script type="text/javascript">

window.location = "login.php"

</script>
</html>
<?php
}}
?>
<html>
<link rel="stylesheet" type="text/css" href="style.css">
<body>

</body>

Question 3

The problem is in your protected.php file you have written a wrong condition your code has ($iuser !== $usr it should be if ($iusr !== $usr || $ipass !== $pass) { also condition $ipass !== $ipass should be like $ipass !== $pass below is the updated protected.php file

<?php
$usr = "admin";
$pass = "admin";

$iusr = $_POST["user"];
$ipass = $_POST["pass"];

if ($iusr !== $usr || $ipass !== $pass) {
    ?>
    <html>
        <script type="text/javascript">
            window.location = "login.php"
        </script>
    </html>
    <?php
}
?>
<html>
    <link rel="stylesheet" type="text/css" href="style.css">
    <body>
        Login success
    </body>
</html>