Php Debugging from the Trenches
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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.
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Php Debugging from the Trenches
- 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
- 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
- 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
- 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 $title = "Lots of code here!"; // Look Ma, no closing ?> here!
- 26. Obscure error messages • Segmentation fault - issue writing to memory, usually an internal bug • Exception thrown without a stack frame - exception thrown in destructors / exception handlers studio24.net
- 28. Error reporting // Development display_errors = On display_startup_errors = On error_reporting = -1 // E_ALL works PHP5.4+ log_errors = On // Production display_errors = Off display_startup_errors = Off error_reporting = E_ALL log_errors = On
- 29. Error reporting • Any syntax errors in the file that defines error reporting will ignore these settings • Default is to log errors to Apache ErrorLog location, overrides php.ini error_log setting studio24.net
- 30. Custom error handling • set_error_handler() - PHP errors • set_exception_handler() - uncaught exceptions • Log errors • Log stack traces • Display friendly page to users • Use monitoring for alerts studio24.net
- 31. Suppressing errors • @ operator • Don’t do it! • Unless you immediately test the result and deal with it • Suppressed errors still sent to custom error handler • Scream to disable! ini_set('scream.enabled', 1);
- 33. Quick and dirty • var_dump() and print_r() • Very basic, and not that useful • Needs formatting if complex data echo "<pre>";var_dump($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 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 studio24.net
- 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 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
- 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
- 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