This occurs on a FreeBSD server. According to this post[1], I need to install php5-xmlphp5-xml.
# whereis php5-xmlphp5-xml
php5-xml: /usr/ports/textproc/php5-xml
# portmaster textproc/php5-xml
...
# service apache22 restart
[1] http://hi.baidu.com/ericyangbj/item/8289a32771e4171d09750856