[get the size of the output of a php file?]

Is there a way of getting the final size in bytes of the output of a php file?

i have a separate php file that is doing the following...

[code]
print "filesize[" . filesize('roro3.flv') . "]";
[/code]

which outputs...
filesize[18492343]

so that works fine...

I want to get the FINAL size in bytes for the output of a php file... i tried this (which i was sure wouldn't work)

[code]
print "filesize[" . filesize('tracking.php?action=show') . "]";
[/code]

You can see that file for youself here...
http://triotalentagency.com/newsite/tracking.php?action=show

but that code prints out this...

filesize[]

i also tried to use readfile() but that didn't seem to work....
is there anyway to do this?

thanks!