Showing plain PHP code in a HTML page

When PHP echo or print text, it never executes it. That only happens with eval. This means that if you did this:

echo '<?php ... ?>';

it would carry through to the page output and not be parsed or executed.

This means that all you need to do is escape the usual characters (<, >, &, etc.) and you should generally be safe.

Don't reinvent the wheel. I see BBCode in your question. Grab a markdown library and use it instead. SO uses this: http://daringfireball.net/projects/markdown/

1. There is no fear of PHP code injection (unless you are doing some unusual things like eval'ing HTML templates) but always a fear of JS code injection, often called XSS. And all danger coming only from possible JS code.
2. Thus, there is no special treatment for the PHP code, shown on a HTML page. Just treat it as any other data. < > brackets usually being escaped, for obvious reason.
3. Don't reinvent the wheel. PHP has it's highlight_string function for this

If you see escaped quotes on some page, that's most likely because their script escaped them twice (for example magic_quotes did it once, then mysql_query() again). When data sanitisation is done properly, you should not see escape characters in output.