And what did you do to fix it? My one
(running @ redhat6.2, apache1.3.20 + php4.0.6), has the very same problem.
#./apachectl configtest
Syntax error on line 206 of /usr/local/apache/conf/httpd.conf:
Cannot load /usr/local/apache/libexec/libphp4.so into server: /usr/local/apache/libexec/libphp4.so: undefined symbol: gd_module_entry