I install the Domain Mapping plugin on a client site, but it was throwing:
Fatal error: Uncaught Error:  operator not supported for strings in /path/to/site/wp-content/plugins/domain-mapping/classes/Domainmap/Module/Cdsso.php:350
If think that I resolved it by adding the following line just below line 348, however, I'm not sure if that is going to break anything else and it will obviously be blown away when the plugin is updated:
if(!is_array(vars)) $vars = array();
I don't know what is/was causing it, but I would like a better/future-proof solution.