This document summarizes Simon R Jones' presentation on PHP debugging techniques. It discusses common causes of bugs like human error and environmental issues. It also covers PHP errors like syntax errors and warnings. The presentation recommends enabling full error reporting, using tools like Xdebug for stack traces and remote debugging, and following best practices like coding standards and testing. Remote debugging and tools like Charles Proxy can help with debugging AJAX and web services. Legacy code poses unique challenges that refactoring can help address.

1. PHP Debugging from the Trenches
PHP Cambridge, 24 Feb 2014
Simon R Jones, Studio 24

2. Me
•
Founder & Technical Director of
digital agency Studio 24
•
Programming PHP since 1999
•
Contributor to ZF1
•
Contributor to Web Standards
Project
•
Zend Certified Engineer
•
Organiser of PHP Cambridge
and Refresh Cambridge
studio24.net

3. What causes bugs
First steps
PHP errors
Error reporting
Debugging in PHP
Remote debugging
AJAX and Web Services
Legacy software
Good practises
studio24.net

4. “Debugging is a methodical process of finding
and reducing the number of bugs, or defects,
in a computer program or a piece of electronic
hardware, thus making it behave as expected.”
– Wikipedia
studio24.net

5. “Debugging is a methodical process of finding
and reducing the number of bugs, or defects,
in a computer program or a piece of electronic
hardware, thus making it behave as expected.”
– Wikipedia
studio24.net

6. !
We want to avoid this!
WSOD:
White Screen
Of Death!

7. Which makes you feel
like this..

8. While we’d rather be
happily solving problems

9. What causes bugs?
studio24.net

10. Grace Hopper
http://www.history.navy.mil/photos/pers-us/uspers-h/
g-hoppr.htm

11. What causes bugs?
•
Human error (typos)
•
Business logic errors
•
Environmental errors (files, web service, DB)
•
Client-side errors (web browsers)
•
External software bug (PHP, Apache, etc)
•
And sometimes it’s just a feature request or
misunderstanding!
studio24.net

12. First steps
studio24.net

13. “It is a capital mistake to theorize before one
has data. Insensibly one begins to twist facts to
suit theories, instead of theories to suit facts.”
– Sherlock Holmes, A Scandal in Bohemia
studio24.net

14. Understand the issue
1. What did you do?
(how to reproduce the issue)
2. What happened?
(what’s wrong)
3. What was supposed to
happen?
(expected behaviour)
studio24.net

15. Know your environment
•
URL
•
Web browser
•
Operating System / Device
•
JavaScript support?
studio24.net

17. Gather data
•
Logs
•
•
Application logs
•
•
Webserver access / error logs (Nginx, Apache)
Software logs (Varnish, MySQL)
File system
•
Permissions
•
File space, check with df -h
studio24.net

18. Replicate the issue
•
Replay steps
•
Can you replicate it?
•
If not, what’s different?
•
Is the data the same?
•
Are there time-based business rules?
•
Are you using the same web browser/OS?
studio24.net

19. PHP errors
studio24.net

20. PHP errors
•
Parse errors
•
Identified by T_* parser tokens
•
T_PAAMAYIM_NEKUDOTAYIM
issues with static operator ::
•
Sending headers more than once
•
Segmentation faults
•
Exception thrown without a stack frame in
Unknown on line 0. Yay!
studio24.net

21. syntax error, unexpected T_SL …
$heredoc = <<<EOD
My long piece
of text on a few lines
there’s a space here
EOD;
<<<<<<< HEAD
$title = "My updated code";
=======
$title = "My old code";
>>>>>>> master

22. syntax error, unexpected $end in /path/to/file.php on line 27
$heredoc = <<<EOD
My long piece
of text on a few lines
EOD;
there’s a space here
// More code here
for ($x=0; $x<10; $x++) {
// Do stuff
}
echo $something;
but the error reports here

23. Syntax errors
Easily fixed with a decent IDE or running lint before you deploy code:
php -l /path/to/file.php
No syntax errors detected in file.php

24. Warning: session_start() [function.session-start]: Cannot
send session cookie - headers already sent by…
<?php
a space here
$title = "My title";
// More code here
for ($x=0; $x<10; $x++) {
// Do stuff
}
or here will flush the headers
?>

25. Headers sent twice errors
Easily fixed by separating PHP from your templates
and don’t include final ?> in files that only contain PHP
<?php
// Some code
$title = "Lots of code here!";
// Look Ma, no closing ?> here!

26. Obscure error messages
•
Segmentation fault - issue writing to memory, usually an
internal bug
•
Exception thrown without a stack frame - exception
thrown in destructors / exception handlers
studio24.net

27. Error reporting
studio24.net

28. Error reporting
// Development
display_errors = On
display_startup_errors = On
error_reporting = -1 // E_ALL works PHP5.4+
log_errors = On
// Production
display_errors = Off
display_startup_errors = Off
error_reporting = E_ALL
log_errors = On

29. Error reporting
•
Any syntax errors in the file that defines error reporting
will ignore these settings
•
Default is to log errors to Apache ErrorLog location,
overrides php.ini error_log setting
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30. Custom error handling
•
set_error_handler() - PHP errors
•
set_exception_handler() - uncaught exceptions
•
Log errors
•
Log stack traces
•
Display friendly page to users
•
Use monitoring for alerts
studio24.net

31. Suppressing errors
•
@ operator
•
Don’t do it!
•
Unless you immediately test the result and deal with it
•
Suppressed errors still sent to custom error handler
•
Scream to disable!
ini_set('scream.enabled', 1);

32. Debugging in PHP
studio24.net

33. Quick and dirty
•
var_dump() and print_r()
•
Very basic, and not that useful
•
Needs formatting if complex data
echo "<pre>";var_dump($stuff);exit;
•
Xdebug formats var_dump()
studio24.net

34. Displaying errors
Pretty Blue Screen

35. Developer toolbars
•
Send JavaScript messages to console.log()
•
Use Firebug and FirePHP to send messages from PHP to
the console
•
Can profile DB queries via
Zend_Db_Profiler_Firebug

36. Framework debug toolbars
•
Useful for quickly seeing information
•
May slow application down
studio24.net

37. Symfony toolbar

38. Xdebug
•
Stack traces for errors
•
Profiling
•
Remote debugging
•
Enabled via zend_extension in php.ini
•
Don’t use in production!
•
XHProf is designed for profiling on production servers
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39. Xdebug stack trace

40. Remote debugging
studio24.net

41. Xdebug remote debugging
•
Enable in PHP.ini via
xdebug.remote_enable=1
xdebug.remote_port="9000"
1. Set breakpoints
2. Run in browser via session, or browser extension
3. Step through code in IDE
studio24.net

42. Remote debugging in PHP Storm

44. Debugging AJAX and Web Services
studio24.net

45. CURL
•
Great for quickly inspecting headers
curl --HEAD http://domain.com/url
•
Redirects are aggressively cached in most browsers, CURL isn't
•
Use it to debug web services
•
Use Python’s json.tool to format returned JSON
curl -s -H 'X-Auth-Token: AUTH_TOKEN’
-H 'Accept: application/json'
'http://domain.com/url' | python -m json.tool
studio24.net

46. Charles Proxy
•
Records all requests
•
Inspect request and response headers
•
Makes it really easy to debug AJAX
•
You can include just your test domain to reduce amount
of data captured
studio24.net

48. Dealing with SSL
•
Charles acts as a proxy to allow you to inspect SSL requests.
•
This is the same as a man-in-the-middle attack
•
You need to authorise your web browser to allow this
•
Access third-party URLs directly to do this
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49. “Nothing clears up a case so much as stating it
to another person.”
–Sherlock Holmes, The Memoirs of Sherlock Holmes
studio24.net

50. If you’re stuck get a fresh view
•
“Rubber duck” debugging
•
The act of talking through an issue forces
you to think logically
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51. Debugging Legacy software
studio24.net

52. Problems debugging Legacy software
•
“Spaghetti code”
•
No organised class/function system
•
Duplicated code
•
Dead code
•
Global variables
•
Unescaped SQL (and other security woes)
•
Suppressed errors
studio24.net

53. Strategies
•
Ensure you have a local development environment
•
Get the codebase into version control
•
Remove dead code
•
Review error logs
•
Debug with Xdebug to understand code flow
•
Refactor by making small, incremental changes
studio24.net

54. Refactoring
It Was Like That When I Got Here: Steps
Toward Modernizing a Legacy Codebase
http://paul-m-jones.com/archives/2667
!
Modernizing Legacy Applications In PHP
https://leanpub.com/mlaphp
studio24.net

55. Good practises
to help make debugging easier
studio24.net

56. Good practises
•
Use a good IDE (PHPStorm, Zend Studio, NetBeans)
•
Coding standards
•
Document your code
•
Filter In / Escape Out
•
Defensive coding (test all return values)
•
Automated testing
studio24.net

57. PHPUnit
Unit testing

58. Selenium
Browser testing

59. “Chance has put in our way a most singular
and whimsical problem, and its solution is its
own reward”
–Sherlock Holmes, The Adventures of Sherlock Holmes
studio24.net

60. Thanks!
@simonrjones
!
http://www.slideshare.net/simonrjones/TODO

61. Useful links
Environment
http://supportdetails.com/
Browser testing
http://www.browserstack.com/
http://docs.seleniumhq.org/
PHP parser errors
http://php.net/manual/en/tokens.php
Debug toolbars
http://www.sitepoint.com/prettyblue-screen/
https://github.com/zendframework/
ZendDeveloperTools
http://www.firephp.org/
Debugging and Profiling
http://xdebug.org/
https://github.com/facebook/xhprof
https://github.com/perftools/xhgui
Charles Proxy
http://www.charlesproxy.com/
http://techportal.inviqa.com/
2013/03/05/manipulating-http-withcharles-proxy/
PHP Standards
http://www.php-fig.org/
http://www.phptherightway.com/
Refactoring
http://paul-m-jones.com/archives/
2667