Many thanks for the pointers provided here, they did point me in the right direction.

After trying several attempts in building the proper parameters array, I gave up on that approach, so I started playing with XMLwriter as suggested on another post, I could not make it to work either as soapClient requires a conversion to array which is not clean.

I found the best approach for me in this post:

Issue with sending SOAP request, through soapclient, to service that expects CDATA

I completely removed soapClient and used Guzzle instead, at the end it is a lot easier as I can just grab the XML text file I build from SOAPUI and send it directly.