install Zend Debugger:

1. Locate ZendDebugger.so or ZendDebugger.dll file that is compiled for the correct version of PHP (4.3.x, 4.4.x, 5.0.x, 5.1.x, 5.2.x) in the appropriate directory.
2. Add the following line to the php.ini file:
3. 1. Linux and Mac OS X:
      zend_extension=/full/path/to/ZendDebugger.so
      Windows:
      zend_extension_ts=/full/path/to/ZendDebugger.dll
      Windows non-tread safe:
      zend_extension=/full/path/to/ZendDebugger.dll
      (*) the windows non-thread safe is used only with Zend Core 2.0
4. Add the following lines to the php.ini file: zend_debugger.allow_hosts= zend_debugger.expose_remotely=always
5. Place dummy.php file in the document root directory.
6. Restart web server.

How to allow Zend Optimizer and Zend Debugger coexist,
the default suggested configuration was wrong, you must use a hidden arg
zend_extension_manager.debug_server_ts and point it
to a container directory that names its children directory with a name pattern “php-n.m.x”
otherwise you will get:
“PHP Fatal error: [Zend Optimizer] Zend Debugger must be loaded after Zend Optimizer in Unknown on line 0 ”
the following is the steps:

1. cd /full/path/to/ZendDebuger/
2. ln -s 5_m_n_x_comp php-5.m.n.x
3. In php.ini, replace
   zend_extension=/full/path/to/ZendDebugger.so
   with:
   zend_extension_manager.debug_server=/parrent/path/of/php-5.m.n.x
4. 4. restart httpd

as instance:

cd /usr/local/ZendDebugger-5.2.14-linux-glibc23-x86_64/

ln -s 5_2_x_comp php-5.2.x

add the following line to php.ini:
zend_extension_manager.debug_server=/usr/local/ZendDebugger-5.2.14-linux-glibc23-x86_64/
/usr/local/apache2/bin/apachectl start
finished!

Del.icio.us : Zend Optimizer, zend Debugger