The plugin is kicking back a depreciation error, due to user levels having been depreciated, specifically on line 22
add_options_page('Show PHP constants Options'
, 'Show PHP constants'
, 10 , __FILE__
, 'show_php_constants_admin');The 10 needs to be replaced by an admin or super-admin user capability. I used
add_options_page('Show PHP constants Options'
, 'Show PHP constants'
, 'activate_plugins', __FILE__
, 'show_php_constants_admin');