Cannot modify header information

Hello,

I am adding user login and authentication to my website.

I am using following code in checklogin.php file:

try {
login($username, $password);
session_start();
$_SESSION['valid_user'] = $username;
header("Location: webpage1");
}catch (Exception $e){
header("Location: webpage2");
}

I am using login method to validate userid and password from database. After successful login the user goes to webpage1.

At webpage1, before displaying the content I am trying to verify - if the user really logged in. Just to make sure that someone should not access the webpage2 without proper authentication. Therefore, I am adding below code at the start of webpage1.

<phpcode>
<?php
session_start();
if(!session_is_registered(valid_user)){
header("Location: invalid login wbpage");
}
?>
</phpcode>

However, I am getting below error:

Warning:Cannot modify header information - headers already sent by (output started at /home/foldername/websitename/wp-includes/link-template.php:1203) in /home/foldername/websitename/wp-content/plugins/pluginfolder/page.php on line 4

After reading some posts before, I removed any space at the end of page where ?> is present. But still not working.

Can you please help me with this?

Thanks in Advance !