在一个文件中定义的PHP会话值在另一个文件中未定义

Question

I have 2 php files, in the first file:

    <?php
        session_start();    
.
.
.
        $_SESSION['favcolor'] = "green";
        echo "ses: ".$_SESSION['favcolor'];
    ?>

and in the second file:

<?php
    session_start();
    echo "ses: ".$_SESSION['favcolor'];
?>

When executing the first page, i do see the correct favcolor but when executing the second page in the same browser, I get {"error":"Undefined index: favcolor"}. I know I need to check the existence of this key before using it but I was wondering why it's not there in the first place because this is what I want to achieve, the store/restore a session value between PHP calls. What am I missing?