PHP Debugging from the Trenches

PHP Cambridge, 24 Feb 2014 Simon R Jones, Studio 24

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

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What causes bugs

First steps

PHP errors

Error reporting

Debugging in PHP

Remote debugging

AJAX and Web Services

Legacy software

Good practises

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– Wikipedia

“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.”

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– Wikipedia

“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.”

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!

We want to avoid this!

WSOD:White Screen Of Death!

Which makes you feel like this..

While we’d rather be happily solving problems

What causes bugs?

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

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!

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First steps

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– Sherlock Holmes, A Scandal in Bohemia

“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.”

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Understand the issue

1. What did you do?

2. What happened?

3. What was supposed to happen?

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(how to reproduce the issue)

(what’s wrong)

(expected behaviour)

Know your environment

• URL

• Web browser

• Operating System / Device

• JavaScript support?

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Gather data

• Logs

• Webserver access / error logs (Nginx, Apache)

• Application logs

• Software logs (Varnish, MySQL)

• File system

• Permissions

• File space, check with df -h

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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?

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PHP errors

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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!

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syntax error, unexpected T_SL …

$heredoc = <<<EOD My long piece of text on a few lines

EOD;

<<<<<<< HEAD $title = "My updated code"; ======= $title = "My old code"; >>>>>>> master

there’s a space here

syntax error, unexpected $end in /path/to/file.php on line 27

$heredoc = <<<EOD My long piece of text on a few lines

EOD;

// More code here for ($x=0; $x<10; $x++) { // Do stuff }

echo $something;

there’s a space here

but the error reports here

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

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by…

<?php

$title = "My title";

// More code here for ($x=0; $x<10; $x++) { // Do stuff }

?>

a space here

or here will flush the headers

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!

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

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Error reporting

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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

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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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

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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);

Debugging in PHP

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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()

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Displaying errors Pretty Blue Screen

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

Framework debug toolbars

• Useful for quickly seeing information

• May slow application down

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Symfony toolbar

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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Xdebug stack trace

Remote debugging

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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

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Remote debugging in PHP Storm

Debugging AJAX and Web Services

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CURL

• Great for quickly inspecting headers

curl -s -H 'X-Auth-Token: AUTH_TOKEN’ \ -H 'Accept: application/json' \ 'http://domain.com/url' | python -m json.tool

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

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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

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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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–Sherlock Holmes, The Memoirs of Sherlock Holmes

“Nothing clears up a case so much as stating it to another person.”

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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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Debugging Legacy software

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Problems debugging Legacy software

• “Spaghetti code”

• No organised class/function system

• Duplicated code

• Dead code

• Global variables

• Unescaped SQL (and other security woes)

• Suppressed errorsstudio24.net

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

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Refactoring

It Was Like That When I Got Here: Steps Toward Modernizing a Legacy Codebasehttp://paul-m-jones.com/archives/2667!Modernizing Legacy Applications In PHPhttps://leanpub.com/mlaphp

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Good practises to help make debugging easier

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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

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PHPUnit Unit testing

Selenium Browser testing

–Sherlock Holmes, The Adventures of Sherlock Holmes

“Chance has put in our way a most singular and whimsical problem, and its solution is its

own reward”

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@simonrjones!

http://www.slideshare.net/simonrjones/TODO

Thanks!

Useful links

Environmenthttp://supportdetails.com/

Browser testinghttp://www.browserstack.com/ http://docs.seleniumhq.org/

PHP parser errorshttp://php.net/manual/en/tokens.php

Debug toolbars http://www.sitepoint.com/pretty-blue-screen/ https://github.com/zendframework/ZendDeveloperTools http://www.firephp.org/

Debugging and Profilinghttp://xdebug.org/https://github.com/facebook/xhprofhttps://github.com/perftools/xhgui

Charles Proxyhttp://www.charlesproxy.com/ http://techportal.inviqa.com/2013/03/05/manipulating-http-with-charles-proxy/

PHP Standardshttp://www.php-fig.org/ http://www.phptherightway.com/

Refactoringhttp://paul-m-jones.com/archives/2667