By default the OAuth class stores tokens in session variables.
If you want to access the API on behalf of the user when the user is not present, sessions are not the right solution because sessions only exists when the user accesses your Web site.
What you can do is to use a sub-class that stores OAuth tokens in a MySQL database using the mysqli extension.
There is an example script for using this sub-class with Google. You just need to make a few adjustments to make it work with Fitbit:
phpclasses.org/package/7700-PHP-Aut
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Then there is a script that actually accesses the API when the user is not present using tokens stored in a database:
phpclasses.org/package/7700-PHP-Aut
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You should also read this tutorial article that explains how it works.
phpclasses.org/blog/package/7700/po
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