Knowing that:


The Zone of Avoidance (Looking towards the center of the Milky Way) blocks roughly 20%
Each Milky Way star has an angular size, depending on proximity, that obscures a certain percentage of our optical view.
Each Galaxy has an angular size, depending on proximity, that obscures a certain percentage of our optical view.


And so on through Galaxy Clusters, Superstructures, etc...

What percentage of our universe can we not see at visible wavelengths?


Question: Get REQUEST_URI and don't overwrite
Answer: You might try this hack:

if(session_id() == '') 
     session_start();


Should help if WP tries to start session for you.