Be carefule that session_start() will add the followin HTTP header.
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Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
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To prevent it, you need to call session_cache_limiter("nocache") before calling session_start()
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session_cache_limiter("nocache");
session_start();
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There are 4 valuse you can use for session_cache_limiter()

public
private_no_expire	
private
nocache

==> Use "nocache"