Use the Add-on interface to create simple replacements in the content. When a content manage drops your add-on into his content and displays the page, you can include non-dynamic replacement HTML.
One common use for this replacement code is have Contensive add a 'marker' when the content manger drops the add-on. The when Contensive creates the page for display and returns it to your website script, you can programmically replace with something you create in the script.
For instance, you may create a function HelloWorld() in your php page. Then create an Add-on called "HellowWorld" that simply adds the HTML "{{helloworld}}" to the content when it is