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Phpdbg debugger: quickstart guide and practical uses

The release of PHP 5.6 is imminent, and it’s about to bring a little-known debugging tool into the limelight and package repositories everywhere. Here’s a quick-start guide to making phpdbg useful.

phpdbg logo

A note on setup
Although phpdbg is part of the official release, it is bundled in a separate package, like many other PHP components. To install both PHP 5.6 and phpdbg immediately, follow my guide for Fedora. The easier route is to simply wait a few days until the release is official, update your system, then install new package ‘php-dbg’.

What we’ll achieve

1. Open the file for debugging with phpdbg
2. Add a strategic breakpoint
3. View all variables set during paused execution
4. Examine a variable’s current value during paused execution
5. Reassign values to a variable during paused execution
6. Complete code execution and examine the impact of changes on script output

Demo code

We’ll use a brief demo class for showing breaking and debugging procedures. Save this class locally by downloading the script file or copy pasting the script below.


class GreetingClass
	public $fromName = 'Anna';
	public function doGreeting( $toName = null )
		print( 'Hello '.$toName.'! From '.$this->fromName );
$myName = 'Saskia';
$greeter = new greetingClass();
$greeter->doGreeting( $myName );



  1. First of all execute the class file to check the output, for comparison later:
    php -f /path/to/greetingClass.phpOutput: “Hello Saskia! From Anna”
  2. Now start up phpdbg and begin debugging:
    phpdbg -e /path/to/greetingClass.php // leave a space after the -e flag, unlike documentation
  3. add a break point just after the $myName variable has been set:
    b 12 // break on line 12
  4. execute the loaded PHP script
    runYou should see a message saying the breakpoint has been hit (“Hits: 1”)!
  5. Print the class source to terminal, for reference (was the right name Saskia, or Slazenger?):
    l c GreetingClass
  6. Check which variables are currently set:
    info vars
  7. Check the value of var $myName:
    ev $myName
  8. Change the value of $myName, while execution is still paused, to see how it affects the final output:
    ev $myName = 'Yael'
  9. Proceed slowly through the next two script script ‘steps’, execute FETCH_CLASS and METHOD_CALL:
  10. Finish logical execution of the script:
  11. exit and clear the session

Practical uses

The ability to view var values at any point of an executing script is extremely useful. Where is that NULL value being introduced? When are those $_SESSION vars being changed? – hunting down the culprit just became much easier.

Being able to *change* those values is even more important however, especially for large and complex codebases. Seeing what impact a different length string, or longer decimal number has at step 50 of 500 or normal script processing can be extremely time consuming using conventional means, finding how and when the value arries there and how to print it to terminal. Simply adding a break allows unlimited addition and modification of such values however, providing major relief.

Although they’re not covered here, watch points are also settable in phpdbg, and are a very big deal. Check out the official documentation for related commands and examples.


  • Reply Manpreet |

    I’m new to web devlopment and stumbled upon your website as I really need to learn to debug php code for my apps. But I do not have the understanding of the environment that i’ve to create to debug php code.
    Your article is very neat on phpdbg. Could you send me some instructions on how to install phpdbg i mean i need to know everything as I’ve downloaded files from github but dont know what to do with them as there was no .exe file. Also I dont understand how to execute these commands on the command line
    cd /usr/src/php-src/sapi
    git clone https://github.com/krakjoe/phpdbg
    cd ../
    ./buildconf –force
    make -j8
    make install-phpdbg

    what is a SAPI module?

    I’ve installed WAMP server also I have access to web server as well.

    • Reply samtuke |

      Phpdbg was integrated into PHP 5.6 and so you do not have to compile it manually. Also Phpdbg has seen little activity in recent years, so you may want to use an alternative solution.

So, what do you think ?