Two things, and please forgive my lack of precision in my question earlier:
1) the ability to do this seems not to reside in a key (e.g., "b" in addText function ), but in the more generalized TBLSTYLEval key - I suppose this gives you wider latitude.
2) I meant to add bold for one individual cell. In your examples on page http://www.phpdocx.com/api-documentation/word-content/add-table-Word-document-with-PHP, it seems that this ability is limited to a TABLE, and does not extend to the individual cell. It looks like there are some things you can do with individual cells, but BOLD is not one of them.
Am I missing something or is that the way it works?