Hi!
It is not possible, otherwise PHP would not be used anywhere. PHP does include an extension (don't remember) that, when added to the file, makes it display its color-hilighted source instead of parsed html.
Looking at the source of the page is not possible unless the provider hosting PHP files is misconfigured (for example, Apache is not set up to parse php files). In that case, files are directly sent to the client as text.
Best,
Stas
