Php session_start not starting

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PHP

The below snippet occurs when a user enters correct credentials which should start a session and goes to the index.php page. It goes to the index page as expected but it is not starting the session:

Login.php:

if($validUser) {
session_start()
   $_SESSION['id'] = 'Test';
   $_SESSION['username'] = $row['username'];
   header("Location: /index.php");
}

Index.php:

<?php 

echo $_SESSION['id'];
echo $_SESSION['username'];
echo session_status();

?>

Index.php just returns a 1 for the session_status(). What am I missing?

What I have tried:

I haven't really tried anything, I thought the only thing required to start a session is session_start()