I'm using Centos 4.4 with PHP 4.3.9
I can telnet to my client but I get the

"Failed to start debugger session. Please verify if debugger is installed on the server that corresponds to ...."

I checked the apache error log and watched the DBG listener log but nothing.

PHPINFO():

dbg
DBG php debugger, version 2.11.32, Copyright 2001, 2005, Dmitri Dmitrienko, www.nusphere.com

Version 2.11.32
Linked as a shared library.
Profiler compiled, enabled

Directive Local Value Master Value
debugger.enable_session_cookie On On
debugger.enabled On On
debugger.fail_silently On On
debugger.ignore_nops Off Off
debugger.JIT_enabled On On
debugger.JIT_host clienthost clienthost
debugger.JIT_level 3 3
debugger.JIT_port 7869 7869
debugger.profiler_enabled On On
debugger.session_nocache On On
debugger.timeout_seconds 300 300

And
This program makes use of the Zend Scripting Language Engine:
Zend Engine v1.3.0, Copyright (c) 1998-2004 Zend Technologies with DBG v2.11.32, (C) 2000,2005, by Dmitri Dmitrienko


My client has been added to the server allowed host.

I just spent the last two months working with Zend trying to get ZDE to run, I have removed all Zend from my PHP.INI

already answered there
getting-debugger-to-work-t2701.html

I copied the dbg.so-4.3.x to /usr/lib/php4 BUT
In the php.ini I had
extension=dbg.so-4.3.9

so I changed it to:
extension=dbg.so-4.3.x

And now your product works. Why doesn't your error message tell me about the version difference?

It's supposed to do it and so does it when debugger versions like 2.18 or 2.16 are installed. Did not test with 2.11, yet.