PHP Rendering

[J]

I have  a rather strange thing that has always happened and I have 
never hunted down the cause.

I understand fully that when my server is turned off, .php will NOT be 
fully rendered on the pages I am working on. HOWEVER, when making minor 
changes to websites I sometimes just drag the .php page into the 
browser and see the main section so as to check links or text etc., and 
this is sufficient.

HOWEVER, some pages just give me a blank browser, and instead the 
window where the .php file is lights up at me once again as if 
something is telling it to show me where it is. Weird.

How can I get all pages to render in the browser, at least partially? 
Is it something in the code of that section of the page, or is it 
something in a file association?

Thanks,
--J