Howto request customFieldList with TransactionSearchAdvanced (PHP toolkit)

I just keep getting "SearchColumnCustomField is an abstract type and cannot be instantiated"
working off this doco: http://www.netsuite.com/help/helpcenter/en_US/srbrowser/Browser2016_2/schema/other/searchcolumncustomfieldlist.html?mode=package
trying to specify the columns for the search to return, i can do others but not this one. i suspect it has to do with that there can be multiple fields in the field list, so how do i do this ??

this is the code that generates the error:
$search->columns->basic->customFieldList->customField = new SearchColumnCustomField();