Content-Length

Hi I am currently having problems with a custom written web-authentication-proxy. The proxy works fine with standard html pages but not with php pages. After doing a bit of digging I have found that the proxy does a HEAD request and checks the Content-Length to see if the page if valid. I have been able to use Header( "Content-Length: 100" ) to fool the proxy into loading the php page. My question is how do I go about calculating the correct Content-Length for each and every php page on my site. I have done some digging and presume I need to do "Output buffering" and calculate the strlen of the response the php returns, but how do I reference the result of a php page to get a strlen from it?? Any assistance would be welcome Thanks Grant