zend_list_delete undefined symbol

I'm building a server and I configure/make/install
Apache-2.0.54
../configure --enable-so --prefix=/opt/apache2

mysql-standard-5.0.7

php-5.1.0b3
../configure --with-mysql --with-apxs2=/opt/apache2/bin/apxs

WHen I start apache I get an error:
Cannot load /opt/apache2/modules/libphp5.so into server:
/opt/apache2/modules/libphp5.so: undefined symbol: _zend_list_delete

Does anyone have an idea what this means and what I do about it.?

Al