How to verify if the createDocx method done the job correctly, without using file_exists of PHP?
In principle if there is "no fatal error" the docx must have been generated.
A differennt thing is if the document has been "properly generated". Unfortunately the OOXML schema is faulty by design so if one use the standard schema validators you get into circular loops...this is a well known issue.
So teh only solution, by the time being is thorough testing :-)