In general, interfaces follow the same conventions as classes,
with one additional rule: interface names may optionally end in the term,
"Interface", but that term must not be preceded by an underscore. As an example,
Zend_Controller_Plugin_Interface is considered an
invalid name, but Zend_Controller_PluginInterface or
Zend_Controller_Plugin_PluginInterface would be valid
names.
While this rule is not required, it is strongly recommended, as it provides a good visual cue to developers as to which files contain interfaces rather than classes.
Note
This naming convention is new with version 1.9.0 of Zend Framework. Classes that pre-date that version may not follow this rule, but will be renamed in the future in order to comply. See the previous section for more information on the rationale for this change.