Question about mod_rewrite and tracking parameters in a URL

I am having some trouble with the new tracking parameters I started using to measure how effective my social media efforts are. The following works great with Google Analytics but for some reason in my PHP code I am unable to “capture” the value of $utm_source. Is it possible that I need to tweak my mod_rewrite rules to accommodate these extra parameters that are appearing after the question mark? If so, why does this URL work fine when tracking things with Google Analytics?

Thank you!