MySQL 5.1 to MySQL 5.6 update causes php-cgi error: PDO issue?
As noted above, the whole thing was resolved by pushing the default character set back to utf8 from the incompatible (for Drupal, but not for WordPress) utf8mb4...

Relevant my.ini settings as follows:

[client]

default-character-set = utf8 # not utf8mb4

[mysql]

default-character-set = utf8 # not utf8mb4

[mysqld]

# REF: http://stackoverflow.com/a/24487309/3174018
collation-server = utf8_unicode_ci 
character-set-server = utf8 # not utf8mb4