Despite opinions from some of the PHP community, interoperability between PHP applications isn't is such a bad state - at least According to Aaron Wormus:

The fact of the matter is that it's not such a big deal. Porting a plugin from Wordpress to Serendipity is usually trivial, and as we see more web-services based plugins (stuff like the awesome askimet) the task becomes simpler and simpler.

Interoperability is overrated, the more you think of it the more complex it gets. As you start throwing in new factors into the equation (how will this interoperate with a java/c# application) you start to develop monsters like JSR 170 which solve a lot of problems on paper, but in reality are too unwieldy to be truly useful.

He even lists out some of his "steps to interoperability" to help speed things along (like "use as much abstract code as possible" and "use services").