BGOmes,
There's risk of SQL injection and general security regardless the technology, so that cannot be a reason..
If the question is related to templates of landing pages - the best you can do is to include JavaScript which talks to services on your web site, but straightforward php won't work for many reasons.
If the question is related to mail templates, then this is really not and would never be possible.
In case you want to have some really special logic in the templates - go with pages directly from the web site and don't bother with landing pages in Marketo. You can still have your Marketo forms on these pages though.