Oh dear, your application has suddenly stopped working as expected. What should you do now? Using techniques applicable to any php application, we'll go over what to look for and which problems to avoid when trying to determine where the problem lies. We'll show how to correctly identify and deal with problems including: * network connectivity * server config issues * php config * WSOD * common CakePHP application errors

1. EFFECTIVE DEBUGGING
SPEND TIME FIXING PROBLEMS, NOT FINDING THEM
/AndyDawson @AD7six

2. THE WEB IS JUST PLUMBING WITH BYTES
Image:Wikipedia

3. OF COURSE IT'S NOT REALLY THAT SIMPLE
Image:Flickr.com
↖ The hardestproblems to fix existhere.
In the developer's head.

4. FIND IT WITH A HAMMER
Image:Codinghorror.com
//pickone
thrownewException('Madeithere!');
print_r(debug_backtrace());
die(__FILE__.':'.__LINE__);

5. MOST APPS AREN'T THAT SIMPLE
Ahammer willwork, butitmighttake awhile.
Image:energysystemsgroup.com

6. HELLO WORLD EXAMPLE
"Where's mywebpage"

7. CHECK THE HTTP RESPONSE CODE
Is itactually an error?

8. READ THE WEBSERVER ERROR LOG
Location depends on configuration
Typically/var/log/*/
PHPParseerror: [...]in/var/www/example.dev/public/index.phponline4"

9. IDENTIFY THE PROBLEM
$cat/var/www/example.dev/public/index.php
<?php
echo"helloworld';
Parseerrorsareoftenthelinebefore(orearlier)inafile

10. NOT AN ERROR EXAMPLE
Notan error response code so no (direct) logmessages:"

11. GREP FOR IT
$grep-rl"NotFound"*
...
src/Really/Not/Obvious/File.php
$grep-r"NotFound"*
...
src/Really/Not/Obvious/File.php 404=>'NotFound',
$catsrc/Really/Not/Obvious/File.php
...
functionerror(){
die($this->_statusCodes['404']);
}

12. CAKEPHP HELLO WORLD EXAMPLE
"Whythe Four Oh Four?"

13. CHECK THE ERROR LOG
app/tmp/error.log

14. IDENTIFY THE PROBLEM
$catapp/View/Pages/home.ctp
<?php
...
if(!Configure::read('debug')):
thrownewNotFoundException();
endif;

15. DEBUG BASICS
Configand functions everydeveloper should know about

16. PHP CONFIG
Inifile settings are the defaults if notchanged (duh)
display_errors(On/Off)
log_errors(On)
display_startup_errors(Off)
error_reporting(On/Off)
Runtime settings override the inifile -no effectif the file theyare
in has aparse error (duh)
ini_set('display_errors', 0/1)

17. PHP FUNCTIONS
print_r();
debug_backtrace();
get_included_files();
phpinfo();

18. PHP VARIABLES/CONSTANTS
__FILE__
__LINE__
$_SERVER
et.al.

19. A WAY TO REPRODUCE
Have awayto consistentlyreproduce the error
$curl-Ihttp://example.dev/
HTTP/1.1500InternalServerError
Server:nginx
Date:Sat,23Aug201410:33:46GMT
Content-Type:text/html
gitbisect-find regression errors quickly

20. XDEBUG
peclinstallxdebug

21. WEBGRIND
Turn on xdebugprofiling, and look atwhatarequestis doing

22. CLI DEBUGGING
Pause execution with read:
print_r($somethingInteresting);
`readfoo`;
Usefulwhen debuggingaloop.

23. NOT CAKEPHP CODE?
Aghetto debugfunction:
functiondebug($var,$showHtml=null){
if(!defined('DEBUG')||!DEBUG){return;}
if($showHtml===null){
$showHtml=php_sapi_name()==='cli'?false:true;
}
$var=var_export($var,true);
if(!$showHtml){
echo$var;
return;
}
echo'<pre>'.htmlspecialchars($var).'</pre>';
}
createatracefunction(Debugger::trace)tooifneeded

24. LOGIC AIDES
Justvoicingaproblem can find the solution/error
Image:Wikipedia

25. ERROR MESSAGES DON'T LIE
PHP Parse error: syntax error, unexpected '$bar'(T_VARIABLE)
in parse.php on line 3
$catfoo.php
<?php
...
if($foo||bar){
There's an accidentalnone-breakingspace on thatline

26. NETWORKING PROBLEMS
If the problem is noton the webserver -where is it?

27. NAMESERVER PROBLEMS
No response from nameservers is the same as adomain not
existsing
$digcakefest.org
;<<>>DiG9.8.3-P1<<>>cakefest.org
;;globaloptions:+cmd
;;Gotanswer:
;;->>HEADER<<-opcode:QUERY,status:SERVFAIL,id:15266
;;flags:qrrdra;QUERY:1,ANSWER:0,AUTHORITY:0,ADDITIONAL:0
;;QUESTIONSECTION:
;cakefest.org. IN A
;;Querytime:4142msec
;;SERVER:8.8.8.8#53(8.8.8.8)
;;WHEN:TueAug1916:26:592014
;;MSGSIZE rcvd:30
...
;;ANSWERSECTION:
cakefest.org. 1568 IN A 50.56.232.22
...

28. DNS PROBLEMS
"No route to host"means the ip requested isn'taccessible
$traceroutecakefest.org
traceroutetocakefest.org(50.56.232.22),64hopsmax,52bytepackets
1 172.26.81.1(172.26.81.1) 1.032ms 0.912ms 0.912ms
2 192.168.0.1(192.168.0.1) 2.777ms 1.267ms 2.338ms
...
12 rackspace-ic-302090-dls-bb1.c.telia.net(62.115.33.78) 152.340ms 15
9.367ms 146.339ms
13 ***
14 ***
15 ***
If there are stars -there be problems

29. BACKEND SERVER IS
DOWN
502 Bad Gateway
E.g. php-fpm or hhvm is notrunning
Willbe in the webserver error log
There willnotbe anyapplication logentries

30. PITFALLS TO AVOID
Whatnotto do when debuggingcode

31. FIX PROBLEMS, NOT SYMPTOMS
Don'tignore errors/warnings/notices
Fix them in order, some errors willbe the concequence of earlier
problems.

32. COMMON MISTAKES
Notlookingfor error logs
Readingor focussingon the wrongerror message
Notreadingthe error message
Notreadingthe error message aloud
Misinterprettingthe error message
Stoppingdebuggingtoo early.
If "the problem"is aclass/function with source code -debug the
source code of thatfunction
New user: Ifound the problem, it's
app/webroot/index.php!

33. IMPLICIT ASSUMPTIONS
Be waryof implictassumptions
You're debuggingthe same file the browser is loading
You're debuggingthe same application the browser is loading
You're debuggingthe same server the browser is loading
You're debuggingthe same requestthe browser is loading
Conclusions so far are accurate
WTF?Backup, and re-verifyeverything.

34. COMMON PROBLEMS
And how to debug/identifythem

35. NO MODREWRITE
Don'tlook atphp files -the error is atthe webserver.
Enable mod rewrite
Restartthe webserver

36. CAKEPHP AUTOMODELS
$model=ClassRegistry::init('MyModel');
$model->methodName();
SQLError:1064:YouhaveanerrorinyourSQLsyntax;[...]
check[...]fortherightsyntaxtousenear'methodName'
MyModeldoes nothave the function methodName
MyModelhas no behaviour bound implementing
methodName
$modelis an instance of AppModel

37. CAKEPHP AUTOMODELS - IDENTIFICATION
$model=ClassRegistry::init('MyModel');
debug(get_class($model));
##########DEBUG##########
'MyModel'
###########################
$model=ClassRegistry::init('MyModel');
debug(get_included_files());
##########DEBUG##########
array(
...
...app/Model/MyModel.php
...
)
###########################

38. CAN'T FIX IT
(╯°□°）╯︵ ┻━┻
Can'tfind the problem/solution?-gethelp. Butfirst:
Collectthe information you've got
Write astandalone, reproducible example if possible
Reduce the question to it's simplest, form
Don'tover simplifyor make acontrived example
Ask colleagues/the internet
Brace for impact
Stack overflow, the google group and irc as greatplaces to get
help

39. SUMMARY
Identifythe rightpartof arequestto debug
Fix errors in order
Check your assumptions/conclusions ateveryWTF
Formulate aquestion, and Ask for help
Profit!