WP Super Cache is a very popular caching plugin for speeding up WordPress pages. However, due to its nature, some PHP functions may not work properly with WP Super Cache.
In particular, session-related PHP functions such as the is_mobile function and $_SESSION are not compatible with WP Super Cache. This is because these functions and features may return different results each time the page is loaded, and if those results are cached, they may not work as expected.
Specifically, the is_mobile function determines whether the user is accessing the page from a mobile device. If this result is cached, even a visitor from a desktop could be treated as visiting from a mobile device.
Similarly, the $_SESSION variable is used to store visitor-specific data. If this data were cached, all visitors would share the same session data, which could cause unexpected behavior.
Therefore, when using WP Super Cache, care should be taken when using PHP functions such as the is_mobile function and session. When possible, consider using static HTML or JavaScript instead of these functions.