I need to debug PHP. What is my best choice?

XDebug provides step-by-step debugging, and can be used with eclipse PDT, netbeans and even vim. You really should give it a try. There also is Zend Debugger.

You can install PHP IDE with debugging facilities. It will help you to debug your PHP code step-by-step.

Few popular which has this feature:

• Eclipse (See: Debug PHP with Eclipse?)
• NetBeans
• PhpED by NuSphere (Windows only)

For more advanced solution, you can install XDebug extension manually for PHP.

By default when XDebug is loaded, it should show you automatically the backtrace in case of any fatal error. Or you trace into file (xdebug.auto_trace) to have a very big backtrace of the whole request or do the profiling (xdebug.profiler_enable) or other settings. If the trace file is too big, you can use xdebug_start_trace() and xdebug_stop_trace() to dump the partial trace.

Installation

Using PECL:

pecl install xdebug

On Linux:

sudo apt-get install php5-xdebug

On Mac (with Homebrew):

brew tap josegonzalez/phpbrew search xdebugphp53-xdebug

Example of mine configuration:

[xdebug]; Extensionsextension=xdebug.so; zend_extension="/YOUR_PATH/php/extensions/no-debug-non-zts-20090626/xdebug.so"; zend_extension="/Applications/MAMP/bin/php/php5.3.20/lib/php/extensions/no-debug-non-zts-20090626/xdebug.so" ; MAMP; Dataxdebug.show_exception_trace=1       ; bool: Show a stack trace whenever an exception is raised.xdebug.collect_vars = 1             ; bool: Gather information about which variables are used in a certain scope.xdebug.show_local_vars=1            ; int: Generate stack dumps in error situations.xdebug.collect_assignments=1        ; bool: Controls whether Xdebug should add variable assignments to function traces.xdebug.collect_params=4             ; int1-4: Collect the parameters passed to functions when a function call is recorded.xdebug.collect_return=1             ; bool: Write the return value of function calls to the trace files.xdebug.var_display_max_children=256 ; int: Amount of array children and object's properties are shown.xdebug.var_display_max_data=1024    ; int: Max string length that is shown when variables are displayed.xdebug.var_display_max_depth=3      ; int: How many nested levels of array/object elements are displayed.xdebug.show_mem_delta=0             ; int: Show the difference in memory usage between function calls.; Tracexdebug.auto_trace=0                 ; bool: The tracing of function calls will be enabled just before the script is run.xdebug.trace_output_dir="/var/log/xdebug" ; string: Directory where the tracing files will be written to.xdebug.trace_output_name="%H%R-%s-%t"     ; string: Name of the file that is used to dump traces into.; Profilerxdebug.profiler_enable=0            ; bool: Profiler which creates files read by KCacheGrind.xdebug.profiler_output_dir="/var/log/xdebug"  ; string: Directory where the profiler output will be written to.xdebug.profiler_output_name="%H%R-%s-%t"      ; string: Name of the file that is used to dump traces into.xdebug.profiler_append=0            ; bool: Files will not be overwritten when a new request would map to the same file.; CLIxdebug.cli_color=1                  ; bool: Color var_dumps and stack traces output when in CLI mode.; Remote debuggingxdebug.remote_enable=off            ; bool: Try to contact a debug client which is listening on the host and port.xdebug.remote_autostart=off         ; bool: Start a remote debugging session even GET/POST/COOKIE variable is not present.xdebug.remote_handler=dbgp          ; select: php3/gdb/dbgp: The DBGp protocol is the only supported protocol.xdebug.remote_host=localhost        ; string: Host/ip where the debug client is running.xdebug.remote_port=9000             ; integer: The port to which Xdebug tries to connect on the remote host.xdebug.remote_mode=req              ; select(req,jit): Selects when a debug connection is initiated.xdebug.idekey="xdebug-cli"          ; string: IDE Key Xdebug which should pass on to the DBGp debugger handler.xdebug.remote_log="/var/log/xdebug.log" ; string: Filename to a file to which all remote debugger communications are logged.

Specifically, if you're (a newbie like me that found this SO thread after starting out with PHP for the first time and are) trying to figure out what generates the error message in the first place, start with error_log. That's the function that sends a string as "an error message to the web server's error log or to a file."

In general, to learn the fundamentals of error reporting and configuration in PHP, start with the functions in the Error Handling Functions documentation on PHP.net - the comment on that page by petrov dot michael () gmail com is a helpful place to start.

To actually generate the error, your best option as a newbie (meaning, least amount of documentation to read) is to start with print_r. Setting the second argument of print_r to TRUE will return a string, that you can then pass to error_log. This is probably the route that will give you the fastest way to debug, given that your PHP code might be sitting inside some framework that adds in various output control layers (like Wordpress.)