Eclipse用XDebug调试PHP程序

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　　Debugging software is not exactly a fun job for developers. The most widely used debugger for PHP still seems to be a var_dump() statement, possibly in conjunction with die()  to halt program execution at a certain point. While there is nothing wrong using var_dump() statements in  itself, you still need to change the program code to debug a PHP script. And worse,  after you have finished debugging,  you must remove all var_dump() statements again (well you  should, at least). It may well be that a few days later you'll find yourself adding the  very same var_dump() statements to your code again because you need to go hunting another  bug.
　　Of course, you could just comment out the var_dump()  statements, but that looks really ugly in the code. Another option would be to wrap the var_dump() in  conditional clauses, and only execute them when, say, a constant DEBUG is defined. This affects performance, because even if the var_dump()  statements are not executed, the conditional clause must be executed. And besides, it looks even  uglier in the code.
　　As we have already learned in the second articles of this series, having  xdebug create a trace log might be a better option in this case,  because you do not have to change the program code. But a trace log, even if only created for a part of the application,  provides us with a lot of information that may not be necessary  for the debugging process, so using a debugger is a much better  solution. A debugger allows you to pause program execution at any time, inspect or  modify the current variable values, and then continue the program.  By executing a program step by step, you can closely watch the code  doing its job, which hopefully helps you to find out quickly where things go wrong.
　　Beyond var_dump, debugging in PHP has been problematic for a  long time, at least if you were not willing to spend money on a commercial IDE that  supports debugging. With xdebug, open source debugging of PHP code has - in theory - been possible for quite a while now. I say theoretically because until  recently no good and free debug client for xdebug was available for both Windows and Unix.
　　This fall, the problem of having no release-quality cross-platform  xdebug client has been solved with the release of Eclipse PDT. Eclipse PDT is a free IDE for PHP supporting xdebug out of the box. So, without further ado, let us install Eclipse PDT to get started with  debugging.
  Installing Eclipse PDT
　　Eclipse PDT (PDT is short for PHP Development Tools) is written in Java,   and thus works on most platforms where a Java Runtime Environment is  present. If you have no Java Runtime Environment installed on your computer, you  can download your copy from  www.sun.com.  
　　You can download a ready-to-go-package of Eclipse PDT from  http://www.eclipse.org/pdt. You  should choose the appropriate version for your platform, though all packages  are basically only compressed archives of the necessary files to run eclipse. Still, the Windows version comes with an .exe file that starts  Eclipse, which  is way more user-friendly than calling the Java classes directly.
　　As I write this article, the current Release Build version of Eclipse  PDT is R20070917. Choose the latest available version and have some coffee ready, because the Eclipse PDT download is over 100 MB in size. But  since it is a Java application, having a nice cup of coffee while you wait for  the download to complete should be suitable. Once the download has completed, unpack the downloaded archive and you are ready to start Eclipse PDT.
  How debugging works
　　Before we get into the configuration of xdebug and Eclipse PDT, let us  have a look at how PHP debugging with xdebug actually works. This will help you  better understand  the configuration described below.
　　When debugging is enabled in php.ini, xdebug controls the program execution in PHP, which basically means that xdebug can pause and resume  program execution at any time. When program execution is paused, xdebug can retrieve  information about the current program state, like reading variable values. It is even  possible for xdebug to  change the value of a variable, then continue the script execution with a  modified value.
　　The xdebug extension is a server, expecting client connections at a  certain configurable port. There are two protocols that can be used to communicate between the  xdebug client and the xdebug server, GDB and DBGp. GDB is an older protocol, which has  been superceded by  DBGp. By sending commands to the xdebug server, the xdebug client acts  as a sort of  remote control for PHP, telling PHP to pause execution, execute one  step, or to continue the program execution. The client is usually embedded into an editor or the IDE (in our case, into Eclipse PDT), so you will not have to deal with the debug  protocol itself.
　　The PHP server with xdebug can run on a another system than the one  running the xdebug client. That is why xdebug is called a remote debugger. For simplicity, we will  set up the debugging server and client on the same computer.
　　There are two different modes of starting a debug session with xdebug.  They are controlled by the php.ini setting xdebug.remote_mode. The default  setting is req, which makes xdebug always connect to the debug client when a script is  started. If you want xdebug  to only connect to the debug client on a breakpoint or an error in the  script, you can set  xdebug.remote_mode to jit. I would recommend keeping  the default setting,  which you can achieve by putting no xdebug.remote_mode  setting into php.ini.
　　To actually start a debug session, you must pass a parameter XDEBUG_SESSION_START to the script by GET, POST, or cookie. The value of this parameter is  the debug session name, which should be unique at a given point in time, so that xdebug can distinguish  different debug sessions running concurrently. To end a debug session, you need to pass XDEBUG_SESSION_STOP  to the script.
　　Instead of manually dealing with the starting and stopping of debug  sessions, you can install a firefox  plugin  that allows you to conveniently start and stop a debug session with a  mouse click.
　　Using Eclipse PDT, you will not even have to worry about the browser  plugin, as the IDE takes care of passing the appropriate parameters to the browser.  xdebug also requires setting of a IDE key, which you also do not have to  worry about, because Eclipse sets it for you.
  Configuring xdebug
　　Now let us configure xdebug debugging. Add the following settings to php.ini:
   xdebug.remote_enable=On
xdebug.remote_host="localhost"
xdebug.remote_port=9000
xdebug.remote_handler="dbgp"

　　Make sure you add these lines after the zend_extension_ts  line that loads the xdebug extension. The first entry enables the debugger. The second entry defines that the debug client runs on localhost  - this could be any valid DNS name or IP address, if the client and the server were  not on the same computer.
　　The third setting defines the port the xdebug server expects the client  to listen on (which, strictly speaking, makes the client a server, but let us not get confused about this).  By default, the port is 9000.  This port is configured in Eclipse by default, and if there is no  compelling reason to do otherwise,  you should keep the default port. If you want to change the port, keep  in mind that you have to configure the same port number in Eclipse and php.ini.
　　Also, make sure that no firewall gets into your way. When you start  Eclipse, you might see a warning that Java is trying to set up a server, bind to a port, access  the network, or perform some obscure potentially dangerous action. Of course, it's not dangerous, it's just  the xdebug client trying to listen at port 9000. If you have problems with debugging, check if there are any firewalls  between the debug server and the debug client that might block port 9000.
　　Last but not least, we have to tell xdebug which protocol the client  speaks. Eclipse PDT speaks DBGp, so you may not change this setting.
  Configuring Eclipse PDT
　　To configure Eclipse PDT, start Eclipse by double-clicking the Eclipse  executable.  Create a new PHP project. Let us name the project debug_test. Now, create a file debug.php in the project, add some code, then  save the file.
　　Now let us configure Eclipse for debugging with xdebug. First of all, we will configure Eclipse to launch projects in an external browser  instead of its internal browser. When an external browser is configured, all debugging sessions  will be launched in an external browser as well. Using an external browser is not absolutely necessary, but I prefer to  work with Firefox instead of Eclipse's internal browser. Choose Window from the menu bar, then select Preferences  (see the following screenshot). Expand the General subtree,  and click Web Browser. Now, select the radio button Use  external browser  and click Apply.

　　Eclipse PDT supports the Zend debugger and xdebug. By default, the Zend  debugger is activated. To change this setting to xdebug, expand the PHP  subtree and the Debug subtree of PHP. Then, change PHP debugger to Xdebug and click Apply.
　　Now, choose Run from the menu bar and click the entry Open  Debug Dialog. Then, double click PHP Web Page to create a new debug  configuration.
　　You will see a window with three tabs, Server, Advanced,  and Common. When not already selected, choose Xdebug as Server Debugger. In the File / Project text field, you must enter the path to the  script you want to debug.  This path must be relative to your workspace. On my system, this is /debug_test/debug.php. Click Browse and select the file debug.php in the debug_test  directory.
　　Eclipse needs to know the URL matching the script filename and path you  just entered. This is required to highlight the currently executed line in the source code.  The URL text field shows the URL that is mapped to the filename. By default, the URL text field deactivated because the Auto  Generate checkbox is activated. If the displayed URL does not match the URL you would type into your  browser to see the script you have specified in File / Project, uncheck Auto Generate and enter the  correct URL in the URL text field. If thg script to debug requires any GET parameters, you can append them  to the URL given here.
　　Do not forget to click Apply to save the changes. The following screenshot shows how the debug configuration looks like on  my system:

　　Change to the Advanced tab and make sure that Open in Browser  and the radio button  Debug All Pages both are checked. Now you can close the configuration window and start a debugging  session.
  Debugging a PHP script
　　To start debugging a PHP script, choose Run from the menu bar and  select  Debug. You can shortcut this by pressing the F11 key. Eclipse  will ask you wether you want to open the debug view. You should tick the checkbox to remember this setting,  otherwise you will be asked the same question every time you start debugging.
　　The following screenshot shows Eclipse's debug view of my debug.php  script (which contains some pointless code):

　　Eclipse has opened your system's default browser (or whatever browser  you have configured it to open).  You will not see output in the browser window yet, because Eclipse  by default pauses script execution on the first line of the script, as  if a breakpoint were set on this line. If you want to disable this behaviour, uncheck  the Break at First Line checkbox in the Breakpoint section  from the debug dialog configuration window.
　　As the screenshot shows, you can see the source code of the debugged  file, with the currently executed line marked with an arroq to the left of the line number.  In the above right area, you can choose between different tabs. The Variables tab shows the current values of all variables in the current scope. The superglobal variables are  valid in every scope, so they are always displayed. The Breakpoints tab lets you view and edit all breakpoints in  your script. Eclipse will remember any breakpoints you have set on the code, even if you close and restart Eclipse.
　　You can now continue program execution until the next breakpoint is  reached, execute just one step, or step into  the next function, or out of the next function scope by clicking the  appropriate icons in the Debug  top left area of the window. Stepping is useful when you have located the problem area in your code and want to watch closely what is happening. You will see  the variable values change with each step.
  Changing Variables at Runtime
　　You can even change variable values during script runtime. While this  does not really fix a bug, it  might be useful to provoke certain errors without modifying the source  code. To change a variable, click the current value, modify it and press Enter.
   Breakpoints
　　A breakpoint pauses script execution and allows you to inspect variable  values, then continue the program.  The program execution is also paused if an exception occurs in your PHP  code. To set a breakpoint, right-click a line number in the source code, then  choose Toggle Breakpoints from  the context menu. You can remove breakpoints in the same way, or remove  them in the Breakpoints tab by right-clicking a breakpoint and selecting Remove from the  context menu.
　　You can even add a condition to a breakpoint. Conditional breakpoints  will only pause the program execution when the given condition is met. This is can be very useful when the same piece  of code is executed multiple times with different parametrization. To add a condition to a breakpoint, right-click the breakpoint icon to  the left of the line number in  the source code view. Choose Breakpoint Properties.  You can remove conditions in the same way or by right-clicking a  breakpoint and  selecting Set Condition from the context menu in the Breakpoints  tab.  
　　Check Enable Set Condition and enter a condition in PHP code into  the text field. In my debug.php,  the function test() is called on line 11 and a breakpoint  is set on this line. By adding the condition $a != '' xdebug will only pause program execution on this  line when the local variable $a is non-empty at the time the breakpoint is reached.
　　To end a debugging session, highlight Remote Launch in the top  left pane, then click the Terminate icon which is  located between the Run and the various Step icons. If you  had Eclipse run the script in an external  browser, you need to close the browser window manually.
  Conclusion
　　Remote debugging is a great interactive and non-intrusive way of finding  bugs in your PHP scripts. Instead of putting var_dump() statements into your code, or working your way through a long trace log  including parameter and return values, debugging gives you a macrosopic view of critical areas in your code.