1. Debugging techniques for PHP

• • Jochen Daum

• Automatem Ltd

• PHPUG Auckland meetup 20 Oct 2009

2. Overview

My background

PHP functions

Error and logging set-up

Debugging techniques

The human side of debugging you!

Non-code debugging tools

3. My background

ASP 3.0 experience since 1997

Required website on a CD in 2001

• Luckily can't do that with ASP ;)

Backend programmer for website company Styrofirm in 2002

Build and maintained PHPDbEditTk (Sourceforge) for fast scaffolding

• 4 major systems still in operation and maintenance now

Started Automatem in 2004

4. My (debugging) background

Only had the luxury to build everything from scratch when I was employed with Cabletalk

Other times had to work with other people's code

5. Goals

Debug faster

Debug for solving problems

• Understanding code is not the goal

6. PHP facilities overview

echo() / var_dump() / print_r() / die ()

debug_backtrace ()/debug_print_backtrace()

get_defined_vars () / get_declared_classes() / method_exists()

Error display and logging

isset()

7. echo()

Echo($var) prints out $var

Prints Array for arrays

(Catchable) fatal error for objects

Don't use for debugging, waste of time.

8. var_dump() / print_r()

Prints variable and its type

Works with all types including arrays, objects, resources

Unlimited depth

Shows references

var_dump print_r output differences

9. var_dump()/print_r() by mail()

Useful when debugging a live site

var_dump() needs use of output buffering

print_r can return value: print_r($var,true);

10. debug_backtrace()

Prints stack trace of function calls with variables passed

Shows file, line number, class, object, passed variables

Can easily get messy (see example)

Returns array. print_debug_backtrace() prints right away, but also messy

11. my_debug_backtrace()

Prefer to see only line numbers

my_debug_backtrace() (for code see http://nz.php.net/manual/en/function.debug-backtrace.php#85915 )

12. Error types

http://nz.php.net/manual/en/errorfunc.constants.php

• E_ERROR Fatal run-time errors. Script stops.

• • Run out of memory

• • using array as object etc

• E_PARSE Compile-time parse errors.

• • Syntax errors

• E_WARNING Run-time warnings

• • Database connection failed, division by 0

• 4E_NOTICE (integer) Notices

13. Error Types (2)

http://nz.php.net/manual/en/errorfunc.constants.php

• E_WARNING Run-time warnings

• • Database connection failed

• • Division by 0

• 4E_NOTICE (integer) Notices

• • Variable not declared

• • Typo

• • Array index missing

14. Error Logging

Can be configured in php.ini or through ini_set() (nearly all of them)

display_errors: 0,1,'stderr' (fcgi only)

• Should be disabled on live site

• Enabled on development site,otherwise no error shown at all!

• Doesn't affect Fatal Errors

15. Error Logging (1)

log_errors: 0,1 to log errors to a file

error_log: 'syslog', file_path

• File to send logs to

log_errors_max_len, ignore_repeated_errors, ignore_repeated_source

• Configuration options of how errors get logged

16. Debugging techniques

Backtracing

Divide and conquer

17. Back tracing

Start where output is created:

• Actual html, json or Xml output

• Error message/ fatal error

Go back step by step until the source is found, for example of:

• A variable value

• A problem

Example: backtrace through Joomla site

18. Divide and conquer

Assume that code is a black box, you don't need to understand it

Look at debug_backtrace

Start debugging in the middle of the list of functions.

Work your way either way of the code.

19. My code guidelines

...as they relate to debugging

• Have a coding style and be consistent

• • Indenting

• • Where the brackets go

• • directory/file structure

• E_NOTICE on!

• • Takes a bit of work, but worth it

20. My code guidelines (2)

No if without else

• the code will never go here - are you sure?

No switch without default

No foreach without if

21. My code guidelines (3)

Reduce lines of code

• Use libraries

• Centralise everything

Don't change if not broken

22.

Development IDE

Breakpoint

Conditional breakpoint

Debugging: Text Editor vs. IDE

Text Editor

var_dump() or var_dump();die();

If ($cond){ var_dump();//die(); }

23. Debugging: Text Editor vx. IDE

Development IDE

Stack trace

Inspect array/object value

Text Editor

print_r(debug_backtrace());//die();

Complicated/ lots of var_dump();

24. Debugging: Text Editor vs. IDE

Development IDE

Watch Variable/ Expression

Text Editor

Lots of (!) var_dump()

25. Human aspects of debugging

Tiredness

Confidence

Focus

26. Human aspects of debugging

Tiredness

• Get some sleep

• Be wiling to put problem aside

27. Human aspects of debugging

Confidence

• Generally start with smaller projects

• Plan more time if you solve a certain kind of problem first time

• Helps to be familiar with some frameworks and CMS systems: you will notice how they all use similar approaches

28. Human aspects of programming

Focus

• Debugging requires concentration

• If you won't have time at your hand, don't start going into something

29. Other debugging tools

CVS/ Subversion/ Git/ Diff

Firebug net console/ Fiddler/ Wireshark

FirePHP

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