On my phpBB3 board i include some code the uses :
set_error_handler('functionname', E_ALL ^ E_WARNING);
error_log(...);

... to save errors to a logfile.

When doing so i get alot of error messages like :

line 165 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: S_IN_SEARCH ]
line 171 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: SEARCH_WORDS ]
line 184 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: U_EMAIL_PM ]
line 184 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: U_PRINT_PM ]
line 207 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: S_SHOW_COPPA ]
line 207 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: S_REGISTRATION ]
line 10 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_FORUM_RULES ]
line 67 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_HAS_POLL ]
line 188 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_PROFILE_FIELD1 ]
line 138 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_FIRST_ROW ]
line 188 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_PROFILE_FIELD1 ]
line 138 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_FIRST_ROW ]
line 188 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_PROFILE_FIELD1 ]
line 138 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_FIRST_ROW ]
line 188 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_PROFILE_FIELD1 ]
line 138 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_FIRST_ROW ]
line 188 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_PROFILE_FIELD1 ]
line 138 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_FIRST_ROW ]
line 188 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_PROFILE_FIELD1 ]
line 138 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_FIRST_ROW ]
line 188 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_PROFILE_FIELD1 ]
line 138 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_FIRST_ROW ]
line 188 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_PROFILE_FIELD1 ]
line 138 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_FIRST_ROW ]
line 188 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_PROFILE_FIELD1 ]
line 138 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_FIRST_ROW ]
line 188 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_PROFILE_FIELD1 ]
line 214 in /forum/cache/tpl_prosilver_viewtopic_body.html.php [ Undefined index: S_QUICK_REPLY ]
line 13 in /forum/cache/tpl_prosilver_jumpbox.html.php [ Undefined index: S_DISPLAY_JUMPBOX ]
line 10 in /forum/cache/tpl_prosilver_overall_footer.html.php [ Undefined index: S_WATCH_FORUM_LINK ]
line 165 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: S_IN_SEARCH ]
line 184 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: U_EMAIL_TOPIC ]
line 184 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: U_EMAIL_PM ]
line 184 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: U_PRINT_TOPIC ]
line 184 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: U_PRINT_PM ]
line 5 in /forum/cache/tpl_prosilver_search_results.html.php [ Undefined index: SEARCH_TOPIC ]
line 149 in /forum/cache/tpl_prosilver_search_results.html.php [ Undefined index: PREVIOUS_PAGE ]
line 149 in /forum/cache/tpl_prosilver_search_results.html.php [ Undefined index: NEXT_PAGE ]
line 1 in /forum/cache/tpl_prosilver_jumpbox.html.php [ Undefined index: S_VIEWTOPIC ]
line 4 in /forum/cache/tpl_prosilver_jumpbox.html.php [ Undefined index: S_VIEWFORUM ]
line 7 in /forum/cache/tpl_prosilver_jumpbox.html.php [ Undefined index: SEARCH_TOPIC ]
line 13 in /forum/cache/tpl_prosilver_jumpbox.html.php [ Undefined index: S_DISPLAY_JUMPBOX ]
line 10 in /forum/cache/tpl_prosilver_overall_footer.html.php [ Undefined index: S_WATCH_FORUM_LINK ]
line 10 in /forum/cache/tpl_prosilver_overall_footer.html.php [ Undefined index: U_WATCH_TOPIC ]
line 10 in /forum/cache/tpl_prosilver_overall_footer.html.php [ Undefined index: U_BOOKMARK_TOPIC ]
line 10 in /forum/cache/tpl_prosilver_overall_footer.html.php [ Undefined index: U_BUMP_TOPIC ]
line 165 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: S_IN_SEARCH ]
line 171 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: SEARCH_WORDS ]
line 184 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: U_EMAIL_TOPIC ]
line 184 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: U_EMAIL_PM ]
line 184 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: U_PRINT_TOPIC ]
line 184 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: U_PRINT_PM ]
line 207 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: S_SHOW_COPPA ]
line 207 in /forum/cache/tpl_prosilver_overall_header.html.php [ Undefined index: S_REGISTRATION ]
line 39 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: CLICKS ]
line 55 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_LAST_ROW ]
line 7 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_FIRST_ROW ]
line 39 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: CLICKS ]
line 55 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_LAST_ROW ]
line 7 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_FIRST_ROW ]
line 39 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: CLICKS ]
line 55 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_LAST_ROW ]
line 7 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_FIRST_ROW ]
line 39 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: CLICKS ]
line 55 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_LAST_ROW ]
line 7 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_FIRST_ROW ]
line 39 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: CLICKS ]
line 55 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_LAST_ROW ]
line 7 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_FIRST_ROW ]
line 39 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: CLICKS ]
line 55 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_LAST_ROW ]
line 7 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: S_FIRST_ROW ]
line 39 in /forum/cache/tpl_prosilver_forumlist_body.html.php [ Undefined index: CLICKS ]
line 10 in /forum/cache/tpl_prosilver_overall_footer.html.php [ Undefined index: S_WATCH_FORUM_LINK ]
... and so on

I also wrote about it here : https://www.phpbb.com/community/viewtopic.php?f=46&t=2187883