Debugging With Php

Debugging techniques for PHP Jochen Daum Automatem Ltd PHPUG Auckland meetup 20 Oct 2009

Overview My background PHP functions Error and logging set-up Debugging techniques The human side of debugging – you! Non-code debugging tools

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

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

Goals Debug faster Debug for solving problems Understanding code is not the goal

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()

echo() Echo($var) prints out $var Prints “Array” for arrays (Catchable) fatal error for objects Don't use for debugging, waste of time.

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

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);

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

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 )

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 4 E_NOTICE (integer) Notices

Error Types (2) http://nz.php.net/manual/en/errorfunc.constants.php E_WARNING Run-time warnings Database connection failed Division by 0 4 E_NOTICE (integer) Notices Variable not declared Typo Array index missing

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

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

Debugging techniques Backtracing Divide and conquer

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

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.

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

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

My code guidelines (3) Reduce lines of code Use libraries Centralise everything Don't change if not broken

Development IDE Breakpoint Conditional breakpoint Debugging: Text Editor vs. IDE Text Editor var_dump() or var_dump();die(); If ($cond){ var_dump();//die(); }

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();

Debugging: Text Editor vs. IDE Development IDE Watch Variable/ Expression Text Editor Lots of (!) var_dump()

Human aspects of debugging Tiredness Confidence Focus

Human aspects of debugging Tiredness Get some sleep Be wiling to put problem aside

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

Human aspects of programming Focus Debugging requires concentration If you won't have time at your hand, don't start going into something

Other debugging tools CVS/ Subversion/ Git/ Diff Firebug net console/ Fiddler/ Wireshark FirePHP Issue tracker

Debugging With Php

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