When I compile php (libxml2.2.8 is installed), the compile fails due to --with-dom option. Here is the output given to me during compile:

./configure \
   --with-apxs=/usr/www/bin/apxs \
   --disable-debug \
   --enable-safe-mode \
   --enable-trans-sid \
   --enable-track-vars \
   --with-mysql=/usr/local/mysql \
   --with-pgsql=/usr/local/pgsql \
   --with-magic-quotes \
   --with-dom=/usr/local

<snip>

make[2]: Leaving directory `/src/php-4.0.3pl1/main'
make[1]: Leaving directory `/src/php-4.0.3pl1/main'
Making all in ext
make[1]: Entering directory `/src/php-4.0.3pl1/ext'
Making all in domxml
make[2]: Entering directory `/src/php-4.0.3pl1/ext/domxml'
make[3]: Entering directory `/src/php-4.0.3pl1/ext/domxml'
/bin/sh /src/php-4.0.3pl1/libtool --silent --mode=compile gcc 
-I.
-I/src/php-4.0.3pl1/ext/domxml
-I/src/php-4.0.3pl1
-I/src/php-4.0.3pl1/main
-I/usr/www/include
-I/src/php-4.0.3pl1/Zend
-I/src/php-4.0.3pl1
-I/usr/local/include
-I/usr/local/mysql/include
-I/src/php-4.0.3pl1/ext/xml/expat/xmltok
-I/src/php-4.0.3pl1/ext/xml/expat/xmlparse
-I/src/php-4.0.3pl1/TSRM

-D_POSIX_PTHREAD_SEMANTICS -DEAPI -DXML_BYTE_ORDER=21 -g -O2
 -c php_domxml.c
php_domxml.c: In function `php_minit_domxml':
php_domxml.c:162: `XML_GLOBAL_NAMESPACE' undeclared (first use in this function)
php_domxml.c:162: (Each undeclared identifier is reported only once php_domxml.c:162: for each function it appears in.)
make[3]: *** [php_domxml.lo] Error 1
make[3]: Leaving directory `/src/php-4.0.3pl1/ext/domxml'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/src/php-4.0.3pl1/ext/domxml'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/src/php-4.0.3pl1/ext'
make: *** [all-recursive] Error 1