Debugging in PHP
PHPCAMP 2008, Pune
20th Sept.
Jignesh Thummar
jignesh.thummar@gmail.com

Debugging
“Debugging is twice as hard as writing the
code in the first place. Therefore, if you
write the code as cleverly as possible, you
are, by definition, not smart enough to
debug it.”
- Brian W. Kernighan, father of UNIX.

Why do I need debugger?
●
Good programmer always use debugger
●
Are echo() and var_dump() enough capable?
●
What about some complex program?

Available debuggers for PHP
APD (Advanced PHP Debugger)
- http://pecl.php.net/package/apd
- open source, but unmaintained
Zend Debugger
- http://www.zend.com/en/products/studio/
- commercial
DBG
- http://dd.cron.ru/dbg/
- half commercial
Xdebug

Introducing Xdebug
●
Xdebug – An open source debugging tool
●
Developed by Derick Rethans – core PHP developer
●
About 4 years old
●
Current version 2.0.3
●
Works on Linux (at least), Windows and Mac

Installation
with PECL
#pecl install xdebug
without PECL
#wget http://xdebug.org/files/xdebug-2.0.3.tgz
#tar -zxvf xdebug-2.0.3
#cd xdebug-2.0.3
#phpize
#./configure
#make
#make install
#cp modules/xdebug.so /path/to/your/ext/directory
Debian users:
# sudo apt-get install php5-xdebug

Enabling Xdebug
in php.ini add: zend_extension=xdebug.so
for Windows users
●
download .dll from http://pecl4win.php.net
●
in php.ini add:
zend_extension_ts=C:\\php\\ext\\xdebug.dll
restart apache
Note: Zend's extensions (optimizer, debugger,
cache) prohibit Xdebug (and other non-Zend zend-
extensions) from loading

What is there in xdebug?
xdebug provides
●
stack traces and call traces
●
profiling information of the script
●
code coverage analysis
●
remote debugging
●
memory allocation, time tracking for function calls
●
error/warning/notice messages more explainable

PHP/Webserver is crashing?
●
Really?
●
When?
stack overflow
infinite recursion
●
Need to protect it?

Protection
<?php
function myfunction()
{
myfunction();
}
myfunction();
?>
xdebug.max_nesting_level = 4

debug the variables
●
xdebug.show_local_vars=on
●
Error display
for superglobals
●
xdebug.dump.COOKIE=*
●
xdebug.dump.POST=login,useremail
●
xdebug.dump.SESSION=sess_id,hash
●
same for GET, FILES, SERVER, ENV, REQUEST
demo

color coded variables
override var_dump() by xdebug_var_dump()
xdebug.var_display_max_children
xdebug.var_display_max_data
xdebug.var_display_max_depth
demo

few important functions
xdebug_memory_usage( )
xdebug_peak_memory_usage( )
xdebug_time_index( ) in seconds
xdebug_enable()
xdebug_disable()
xdebug_call_class()
xdebug_call_function()
xdebug_call_file()
xdebug_call_line()

Execution Trace
●
new to the team?
●
team member left the project and you don't know what is
happening in the script?
●
want to know the flow of the script
●
or want to do function traces
●
show the value passed to functions/methods and also
return values
* ini options
xdebug.auto_trace = 0
xdebug.trace_format =0
xdebug.trace_output_dir = /tmp/xdebug-traces
xdebug.trace_options = 0
xdebug.trace_output_name = trace.%c
// crc32, timestamp, pid, %r, scriptname etc.

more options for trace
●
xdebug.collect_params=0 // can make it 1,2,3,4
●
xdebug.collect_return=1
●
xdebug.collect_includes=on
●
xdebug.show_mem_delta=1
functions for trace
xdebug_start_trace(string trace_file [,integer options])
xdebug_stop_trace()
xdebug_get_tracefile_name( )
demo

Code Coverage
Which code snippet do I use in my script?
add in php.ini - xdebug.extended_info = 1
xdebug_start_code_coverage([int option])
XDEBUG_CC_UNUSED -
XDEBUG_CC_DEAD_CODE -
var_dump(xdebug_start_code_coverage())
xdebug_stop_code_coverage()
demo

Profiler
●
functionality to analyze the performance of the script
●
find out bottleneck in the script
●
will give output in cachegrind compatible file
viewable in
KcacheGrind - KDE, Linux
webgrind - http://code.google.com/p/webgrind
WinCacheGrind – Windows users
demo

analyze running script
DBGp – debugger protocol
xdebug.remote_enable=1
xdebug.remote_handler=dbgp
xdebug.remote_mode=req
xdebug.remote_host=127.0.0.1
xdebug.remote_port=9000
xdebug.idekey=Jignesh
xdebug.remote_log = /tmp/xdebug_remote_log

need to install debugclient
go to xdebug source directory
# cd debugclient
# ./buildconfig
# ./configure --with=libedit
# make
# make install
Editor supports :
PDT(PHP Eclipse), VIM, Komodo, netbeans and many
more
demo it

Resources
www.xdebug.org
www.derickrethans.nl

Thanks
Happy debugging :)