Add the "-s" switch as an option. 

Another very useful option of phpcs is the -s switch.
This allows to print the rule name (sniff code) on all reports.

For example:
--------------------------------------------------------------------------------
FOUND 1 ERROR(S) AFFECTING 1 LINE(S)
--------------------------------------------------------------------------------
 1 | ERROR | The PHP open tag does not have a corresponding PHP close tag
   |       | (Generic.PHP.ClosingPHPTag.NotFound)
--------------------------------------------------------------------------------

With the -s switch, we can easily know that the actual rule that threw the error is "Generic.PHP.ClosingPHPTag.NotFound"