We’ve all encountered the issue: a blank screen or worse, a 500 error on the server. How do you cope with this kind of issue, what do you do to find the culprit and resolve the error.

1. D E B U G G I N G W O R D P R E S S
B R E C H T RY C K A E R T

2. B R E C H T
RY C K A E R T
• Works at combell.com
• WP user since 1.5
• Passionate about
WordPress Security &
Performance
@brechtryckaert
brechtryckaert.com

3. W H E N L A U N C H I N G
Y O U R N E W W E B S I T E

4. O R W H E N U P D AT I N G Y O U R
P L U G I N S O R T H E M E S

5. … O R E V E N M I G R AT I N G
Y O U R W E B S I T E …

8. W H O ’ S E X P E R I E N C E D
T H I S B E F O R E ?

9. D O E S N ’ T I T M A K E Y O U
F E E L L I K E …

11. T W O K I N D S O F
E R R O R C O D E S …

12. 4 0 0 - C O D E S
5 0 0 - C O D E S

13. 4 0 0 - C O D E S
5 0 0 - C O D E S
You stuffed up (client errors)
Your server stuffed up (server errors)

14. 400 Bad Request
401 Unauthorized (RFC 7235)
402 Payment Required
403 Forbidden
404 Not Found
405 Method Not Allowed
…

15. 400 Bad Request
401 Unauthorized (RFC 7235)
402 Payment Required
403 Forbidden
404 Not Found
405 Method Not Allowed
…

16. 500 Internal Server Error
501 Not Implemented
502 Bad Gateway
503 Service Unavailable
504 Gateway Timeout
…

17. 500 Internal Server Error
501 Not Implemented
502 Bad Gateway
503 Service Unavailable
504 Gateway Timeout
…
D O N ’ T A S K M E .
I ’ M C L U E L E S S ; - )

18. H O W T O D E B U G ?

19. - S E R V E R L O G S
- W P _ D E B U G
- W P - C L I
- S C R I P T _ D E B U G
- S AV E Q U E R I E S
- P L U G I N S

20. S E R V E R L O G S

21. T Y P E S O F E R R O R S I N T H E L O G S
• FATAL ERROR
• WARNING/NOTICE
• LIMIT-RELATED

22. [Sat Feb 10 02:05:22.490389 2018] [error] [pid 5740] mod_proxy_fcgi.c(954): (70007)The timeout specified has expired: [client
141.101.77.82:32531] AH01075: Error dispatching request to : (polling)
[Sat Feb 10 03:42:55.356581 2018] [error] [pid 34419] mod_authz_core.c(873): [client 176.62.169.28:27736] AH01630: client
denied by server configuration: /data/sites/web/somewebsite.com/.user.ini
[Sat Feb 10 10:50:24.838424 2018] [error] [pid 41100] mod_proxy_fcgi.c(954): (70007)The timeout specified has expired: [client
162.158.111.75:24677] AH01075: Error dispatching request to : (polling)
[Sat Feb 10 10:50:36.741167 2018] [error] [pid 48392] apache2_util.c(271): [client 172.68.65.254:35087] [client 172.68.65.254]
ModSecurity: Access denied with code 403 (phase 2). Matched phrase "SemrushBot" at REQUEST_HEADERS:User-Agent.
[file "/usr/local/httpd/conf/modsecurity-crs/activated-rules/modsecurity_crs_35_bad_robots.conf"] [line "20"] [id "990002"]
[rev "2"] [msg "Request Indicates a Security Scanner Scanned the Site"] [data "mozilla/5.0 (compatible; semrushb
ot/1.2~bl; +http://www.semrush.com/bot.html)"] [severity "CRITICAL"] [ver "OWASP_CRS/2.2.9"] [maturity "9"] [accuracy
"9"] [tag "OWASP_CRS/AUTOMATION/SECURITY_SCANNER"] [tag "WASCTC/WASC-21"] [tag "OWASP_TOP_10/A7"] [tag
"PCI/6.5.10"] [h
ostname “www.somewebsite.com”] [uri "/tag/romeinen/"] [unique_id "Wn7AbE4XLPnYH7fIxTBlGwAAAcY"]

23. [Sat Feb 10 02:05:22.490389 2018] [error] [pid 5740] mod_proxy_fcgi.c(954): (70007)The timeout specified has expired: [client
141.101.77.82:32531] AH01075: Error dispatching request to : (polling)
[Sat Feb 10 03:42:55.356581 2018] [error] [pid 34419] mod_authz_core.c(873): [client 176.62.169.28:27736] AH01630:
client denied by server configuration:/data/sites/web/somewebsite.com/.user.ini
[Sat Feb 10 10:50:24.838424 2018] [error] [pid 41100] mod_proxy_fcgi.c(954): (70007)The timeout specified has expired: [client
162.158.111.75:24677] AH01075: Error dispatching request to : (polling)
[Sat Feb 10 10:50:36.741167 2018] [error] [pid 48392] apache2_util.c(271): [client 172.68.65.254:35087] [client 172.68.65.254]
ModSecurity: Access denied with code 403 (phase 2). Matched phrase "SemrushBot" at REQUEST_HEADERS:User-Agent.
[file "/usr/local/httpd/conf/modsecurity-crs/activated-rules/modsecurity_crs_35_bad_robots.conf"] [line "20"] [id "990002"]
[rev "2"] [msg "Request Indicates a Security Scanner Scanned the Site"] [data "mozilla/5.0 (compatible; semrushb
ot/1.2~bl; +http://www.semrush.com/bot.html)"] [severity "CRITICAL"] [ver "OWASP_CRS/2.2.9"] [maturity "9"] [accuracy
"9"] [tag "OWASP_CRS/AUTOMATION/SECURITY_SCANNER"] [tag "WASCTC/WASC-21"] [tag "OWASP_TOP_10/A7"] [tag
"PCI/6.5.10"] [h
ostname “www.somewebsite.com”] [uri "/tag/romeinen/"] [unique_id "Wn7AbE4XLPnYH7fIxTBlGwAAAcY"]

24. [Sat Feb 10 02:05:22.490389 2018] [error] [pid 5740] mod_proxy_fcgi.c(954): (70007)The timeout specified has expired: [client
141.101.77.82:32531] AH01075: Error dispatching request to : (polling)
[Sat Feb 10 03:42:55.356581 2018] [error] [pid 34419] mod_authz_core.c(873): [client 176.62.169.28:27736] AH01630: client
denied by server configuration: /data/sites/web/somewebsite.com/.user.ini
[Sat Feb 10 10:50:24.838424 2018] [error] [pid 41100] mod_proxy_fcgi.c(954): (70007)The timeout
specified has expired: [client 162.158.111.75:24677]
AH01075: Error dispatching request to : (polling)
[Sat Feb 10 10:50:36.741167 2018] [error] [pid 48392] apache2_util.c(271): [client 172.68.65.254:35087] [client 172.68.65.254]
ModSecurity: Access denied with code 403 (phase 2). Matched phrase "SemrushBot" at REQUEST_HEADERS:User-Agent.
[file "/usr/local/httpd/conf/modsecurity-crs/activated-rules/modsecurity_crs_35_bad_robots.conf"] [line "20"] [id "990002"]
[rev "2"] [msg "Request Indicates a Security Scanner Scanned the Site"] [data "mozilla/5.0 (compatible; semrushb
ot/1.2~bl; +http://www.semrush.com/bot.html)"] [severity "CRITICAL"] [ver "OWASP_CRS/2.2.9"] [maturity "9"] [accuracy
"9"] [tag "OWASP_CRS/AUTOMATION/SECURITY_SCANNER"] [tag "WASCTC/WASC-21"] [tag "OWASP_TOP_10/A7"] [tag
"PCI/6.5.10"] [h
ostname “www.somewebsite.com”] [uri "/tag/romeinen/"] [unique_id "Wn7AbE4XLPnYH7fIxTBlGwAAAcY"]

25. Tue Jan 16 15:10:55.316779 2018] [error] [pid 6002] mod_proxy_fcgi.c(848): [client
178.119.78.83:63467] AH01071: Got error 'PHP message: PHP Fatal error: Uncaught Error: Call to
undefined method SitePress::is_display_as_translated_taxonomy() in /data/sites/web/somewebsite/
www/wp-content/plugins/woocommerce-multilingual/inc/class-wcml-dependencies.php:365nStack
trace:n#0 /data/sites/web/somewebsite/www/wp-includes/class-wp-hook.php(286):
WCML_Dependencies->check_wpml_config('')n#1 /data/sites/web/somewebsite/www/wp-includes/
class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array)n#2 /data/sites/web/somewebsite/
www/wp-includes/plugin.php(453): WP_Hook->do_action(Array)n#3 /data/sites/web/somewebsite/
www/wp-settings.php(450): do_action('init')n#4 /data/sites/web/somewebsite/www/wp-
config.php(83): require_once('/data/sites/web...')n#5 /data/sites/web/somewebsite/www/wp-
load.php(37): require_once('/data/sites/web...')n#6 /data/sites/web/somewebsite/www/wp-admin/
admin-ajax.php(22): require_once('/data/sites/web...')n#7 {main}n thrown in /data/sites/web/
somewebsite/www/wp-content/plugins/woocommerce-multilingua...n'

26. Tue Jan 16 15:10:55.316779 2018] [error] [pid 6002] mod_proxy_fcgi.c(848): [client
178.119.78.83:63467] AH01071: Got error 'PHP message: PHP Fatal error: Uncaught
Error: Call to undefined method SitePress::is_display_as_translated_taxonomy() in /data/sites/web/
somewebsite/www/wp-content/plugins/woocommerce-multilingual/inc/class-wcml-dependencies.php:
365nStack trace:n#0 /data/sites/web/somewebsite/www/wp-includes/class-wp-hook.php(286):
WCML_Dependencies->check_wpml_config('')n#1 /data/sites/web/somewebsite/www/wp-includes/
class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array)n#2 /data/sites/web/somewebsite/
www/wp-includes/plugin.php(453): WP_Hook->do_action(Array)n#3 /data/sites/web/somewebsite/
www/wp-settings.php(450): do_action('init')n#4 /data/sites/web/somewebsite/www/wp-
config.php(83): require_once('/data/sites/web...')n#5 /data/sites/web/somewebsite/www/wp-
load.php(37): require_once('/data/sites/web...')n#6 /data/sites/web/somewebsite/www/wp-admin/
admin-ajax.php(22): require_once('/data/sites/web...')n#7 {main}n thrown in /data/sites/web/
somewebsite/www/wp-content/plugins/woocommerce-multilingua...n'

27. T H E P O W E R O F
W P _ D E B U G

28. W P _ D E B U G
In wp-config.php:
define( 'WP_DEBUG', false );
Enable debugging:
define( 'WP_DEBUG', true );

29. W P _ D E B U G
Extra statements:
define('WP_DEBUG_LOG', true);
Creates logfile in:
/wp-content/debug.log

30. W P _ D E B U G
Prevent public displaying of errors:
define('WP_DEBUG_DISPLAY', false);

31. W P _ D E B U G
Ideal setup:
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);

32. A D D I N G W P - C L I
T O Y O U R T O O L K I T

33. W P - C L I
wp plugin list
wp plugin deactivate
wp plugin activate
wp theme list
wp theme deactivate
wp theme activate
wp checksum core
wp checksum plugin (from version 1.5)

34. W P T H E M E L I S T R E S U LT
Parse error: syntax error, unexpected '$z9973449' (T_VARIABLE) in /data/sites/web/somewebsite/
www/wp-includes/post-template.php on line 1
Warning: array_splice() expects parameter 1 to be array, string given in /data/sites/web/somewebsite/
www/wp-content/plugins/akismet/favicon_316779.ico(105) : eval()'d code(165) : eval()'d code(202) :
eval()'d code on line 206
Warning: Invalid argument supplied for foreach() in /data/sites/web/somewebsite/www/wp-content/
plugins/akismet/favicon_316779.ico(105) : eval()'d code(165) : eval()'d code(202) : eval()'d code on
line 207

35. W P T H E M E L I S T R E S U LT
Parse error: syntax error, unexpected '$z9973449' (T_VARIABLE) in /data/sites/web/somewebsite/
www/wp-includes/post-template.php on line 1
Warning: array_splice() expects parameter 1 to be array, string given in /data/sites/web/somewebsite/
www/wp-content/plugins/akismet/favicon_316779.ico(105) :
eval()'d code(165) : eval()'d code(202) : eval()'d code on line 206
Warning: Invalid argument supplied for foreach() in /data/sites/web/somewebsite/www/wp-content/
plugins/akismet/favicon_316779.ico(105) : eval()'d code(165) : eval()'d code(202) : eval()'d code on
line 207

36. S C R I P T _ D E B U G

37. S C R I P T _ D E B U G
Forces WordPress to use the “dev” versions of some
core CSS and JavaScript files rather than the minified
versions that are normally loaded. This is useful when you
are testing modifications to any built-in .js or .css files.
define( 'SCRIPT_DEBUG', true );

38. S AV E Q U E R I E S

39. S AV E Q U E R I E S
Force WordPress to store information about queries in a
$wpdb array.
define( 'SAVEQUERIES', true );

40. S AV E Q U E R I E S
Visualize by adding this to footer.php:
if ( current_user_can( 'administrator' ) ) {
global $wpdb;
echo '<pre>';
print_r( $wpdb->queries );
echo '</pre>';
}

41. U S E F U L P L U G I N S

42. C O R E
C O N T R O L
• verify crons
• taking manual control of
upgrades
• do HTTP-logging
• Test HTTP-transport-
methods (GET/POST
requests)
https://wordpress.org/plugins/core-control/

43. W P D E B U G
B A R
• Verify queries
• Verify cache
• Verify requests
https://wordpress.org/plugins/debug-bar/

44. T Y P I C A L E R R O R S
A N D H O W T O F I X T H E M

49. Warning: Cannot modify header information - headers already sent by (output started at /data/sites/
web/somewebsite/www/wp-settings.php:84) in /data/sites/web/somewebsite/www/wp-includes/
option.php on line 920

50. Warning: Cannot modify header information - headers already sent by
(output started at /data/sites/web/somewebsite/www/wp-settings.php:84) in /data/sites/web/
somewebsite/www/wp-includes/option.php on line 920

51. H E A D E R S A L R E A D Y S E N T
P O S S I B L E C A U S E S
• Whitespace before <?php or after ?>
• print, echo and other functions producing output
• Raw <html> sections prior <?php code.

52. B A C K T O T H E
( N E A R B Y ) F U T U R E

53. T I D E
https://make.wordpress.org/tide/

54. W H AT C A N Y O U D O ?

55. W H AT C A N Y O U D O ?
Report bugs and issues to the developer

56. Q U E S T I O N S ?

57. T H A N K Y O U !
S L I D E S W I L L B E T W E E T E D O N
@ B R E C H T RY C K A E R T
A N D P U B L I S H E D O N
H T T P S : / / B R E C H T RY C K A E R T. C O M