See bug: http://bugs.php.net/?id=14594 which has good
explanation what the problem is. 

Your response to his error is incorrect.

When I do not correctly specify --with-tsrm-pth, Iget the error "Undefined symbol 'pthread_getspecific'". If I *DO* specify the --with-tsrm-pth correctly, I get, "Undefined symbol 'pthread_mutex_release'".

These are two different errors which are dependent upon whether --with-tsrm-pth was specified correctly or not.

Regardless, I still cannot get PHP-4.2.1 to work with Apache 2.0.

I was having this same problem on Redhat 7.1 trying to install 2.0.36 and PHP 4.2.1. Through a myriad of different installation procedures, I ended up making a change to my sapi/apache2filter/php_functions.c file, installing GNU pth, and adding a lib directory to the root php directory before running 'make install'. After doing all this I got a successful build, but I ended up getting the pth_mutex_release error when trying to start apache. 

My original config line looked like : 

./configure --with-mysql --with-tsrm-pth=/usr/local/libs/pth --with-apxs2=/usr/local/apache2/bin/apx
s --with-xml --enable-track-vars

The --with-tsrm-pth=/usr/local/libs/pth part was recommended in a google search. It was apparently invalid for my install and ultimately the part that caused the pth_mutex_release problem (despite a successful build). After removing the path (and allowing it to use the default), Apache ran fine and PHP works perfectly.

I'm guessing my /usr/local/libs/pth directory is an old version of pth which doesn't handle the 
pth_mutex_release() function in the same way as newer versions. 

Hope this helps..

Try the latest CVS version of PHP.  Since threads are completely broken on FreeBSD, Apache2 defaults to the prefork MPM which is a non-threaded MPM.  So all this stuff is not necessary.  Simply build PHP non-threaded and it will work.  In the latest CVS I added a check for the prefork MPM and turned off threading in PHP.

FreeBsd4.6.2+mysql-3.23.52+httpd-2.0.40+php-4.2.3

1??INSTALL mysql:
tar vxzf mysql-3.23.52.tar.gz
cd mysql-2.23.52
./configure 
--prefix=/path/to/mysql
./make
./make install


2??INSTALL pth-1.4.1:
tar vxzf pth-1.4.1.tar.gz
cd pth-1.4.1
./configure 
(You cann't use "./configure --prefix=/path/to/" !!!)
make
make test
make install

3??INSTALL httpd-2.0.40:
tar vxfz httpd-2.0.40.tar.gz 
cd httpd-2.0.40
./configure ?Cprefix=/path/to/httpd --enable-module=so
make 
make install

4??INSTALL php-4.2.3:
tar vxfz php-4.2.3.tar.gz 
cd ../php-4.2.3
./configure --with-mysql=/path/to/mysql --with-apxs2=/path/to/httpd/bin/apxs
--with-tsrm-pth --with-gd=/usr/local --enable-trans-sid
--enable-track-vars

// ---------if you have't gd ,remove --with-gd=/usr/local

make 
make install 
cp php.ini-dist /usr/local/lib/php.ini

add  httpd.conf :
AddType application/x-httpd-php .php
AddType application/x-httpd-php .php3
AddType application/x-httpd-php-source .phps

//_________ finish all of above,you will phpinfo();....