session_start archive.php vs. single.php not work

If I write in the file archive.php
<?php session_start(); ?> (at the start code)
<?php $_SESSION[$mytext] = ‘100’; ?> (after get_header)

and in single.php file:
<?php session_start(); ?> (at the start code)
<?php echo $_SESSION[$mytext]; ?> (after get_header)

not work;
I not see 100