I'm running Ubuntu 8.04 on this server. I had PHP 5.2 installed via the package manager. I removed it to install PHP 5.3.1 by hand. I built the packages like so

./configure --prefix=/opt/php --with-mysql --with-curl=/usr/bin --with-apxs2=/usr/bin/apxs2
make
make install

This installed PHP 5.3.1 in /opt/php/

$ php -v
PHP 5.3.1 (cli) (built: Dec  7 2009 10:51:14) 
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2009 Zend Technologies

However, when I try to start Apache I get this.

 # /etc/init.d/apache2 restart
 * Restarting web server apache2       
  apache2: Syntax error on line 185 of /etc/apache2/apache2.conf: 
  Syntax error on line 1 of /etc/apache2/mods-enabled/php5.load: 
  Cannot load /usr/lib/apache2/modules/libphp5.so into server: 
  /usr/lib/apache2/modules/libphp5.so: undefined symbol: OnUpdateLong
                                                                     [fail]

Any ideas what's causing this error? All the references I can see have to do with building php5 packages for php4 or the like. PHP4 has never been installed on this machine.