php session

hi people, I'm having a little trouble getting sessions to work properly with php. I have put session_start(); on all pages before any data is printed. The session will start on the first loading of the page but after a link is clicked the session seems to die i have no idea why. for the links on the pages i have included the SID constant like so: <a href='index.php" . SID . "&action=whatever'>Link</a> but after a link is clicked the links them appear to lose the reference to the SID constant so instead of looking like <a href='index.php?PHPSESS=s8743vbkuwivn7&action=whatever>Link</a> they look like <a href='index.php?&action=whatever'>Link</a> the session_start(); isn't right at the start of the pages, its a few lines after but none of the lines before session_start(); print any data, they only store data in variables...does that matter?? I'm completely stumped trying to get this to work properly. can anyone offer any suggestions? Thanks heaps.

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