The session cookie will only be valid for the same domain that generated the cookie/started the session.
Although it will be possible for another php page to pick-up that session, it will only receive the cookie if it is served on the same domain, in which case it is 'part' of your website.
This should therefore not be a problem, because (unless you have a serious problem) only you will be able to add/upload php files to your website.
You should check where the session DATA is saved though. The default 'php' session settings in app/Config/core.php will write the session data to the session save path that is configured in php.ini. This may be a 'shared' directory that is accessible by other websites on the same server.
For better security, set the session configuration in app/config/core.php to 'cake'. This will write the session data to app/tmp/sessions which should only be accessible by your website.