Just seen in the wild on an old-ish Fedora distro.

This works:

extension_dir  = /usr/.../php/extensions/no-debug-non-zts-20060613
extension = apc.so
extension = wbxml.so

Quick check shows good…

$ php -m
[PHP Modules]
apc
ctype
...
wbxml
xdebug
xml


But this does not:

extension = /usr/.../php/extensions/no-debug-non-zts-20060613/apc.so
extension = /usr/.../php/extensions/no-debug-non-zts-20060613/wbxml.so

And yet Zend modules seem to always require a complete path; eg:

zend_extension = /usr/.../php/extensions/no-debug-non-zts-20060613/xdebug.so