HTML source of a php outputted document?

This is a real newbie HTML question, but I just wanted to know before I start to double my work by writing the same thing in two files all the time: I want to display some sample HTML source of a web page that's php3 generated, i.e. not show the php3 source but the html source it generates. The tricky part is that I want to do this on a web page. A horrible solution would be to call a function for each < and >, and if $source is defined output &lt; and &gt; in their place... won't make the code any prettier though. ;) Are there any more elegant solutions?