Append to a php directive rather than replace?

I installed the zendframework package on my开发者_如何学C ubuntu box, and noticed that it added a new ini file at /etc/php5/conf.d/zend.ini with the following:

[Zend]
; include_path=${include_path} ':/usr/share/php/Zend'

When I enable the directive in this file I would expect the resulting include path to be:

.:/usr/share/php:/usr/share/pear:/usr/share/php/Zend

However, all I get is:

 :/usr/share/php/Zend

When I disable the directive, get_include_path() reports:

.:/usr/share/php:/usr/share/pear

Is this valid ini syntax? Why isn't it working?

I'm using PHP version 5.3.2 on Ubuntu 10.04 (last build update May 3rd 2011). It would be great if I could do this, as it keeps the configuration system nice and modular.

Most likely that ${include_path} doesn't contain anything. Check your php.ini for include_path and add your include paths from there.