I don't think that's difficult. You can use querystring. For instance, the .asp pages will do their functionality as needed but when they want to use PHP libraries, they will get redirected to http://www.domain.com/somepage.php?name=<%=myname%>&age=<%=myage%>
The somepage.php will retrive the variables from querystring, pass those variables to the PHP functions and the result will be transfered back to the asp page in a similar way, but with PHP syntax. e.g, http://www.domain.com/somepage.asp&name=&name&age=$age
For the site visitor, he/she won't be able to see any .php page because a redirection will be done on entrance and exit to the PHP page.
I hope you have got what I want to say. Just use your imagination to make usability of PHP functions transparently. Tell me if it works or not.