Hi,
i'm not familiar with Liferay but have some experience in SOAP based integration. (eg. Sharepoint)
As far as I know Liferay Portal can be extended by custom made Portlets coded on PHP or maybe other languages,
so if you need fully functional integration - web services is your way.
You can find more about SDM Web Services integration here:
Technical Reference Guide r12.6
Also it can be integrated via e-mail integration but there is a lot of functional limitations.
Regards,
cdtj