What I tried to do :

Keep a global (CG) array of pointers to the original functions
for the ones which have been disabled
(in the zend_API.c file, zend_disable_function function) ;

Move functions' pointer to their original version for re-enabled ones
(in the mod_php5.c file, send_php function).

But I did not manage to get it working.
And I don't know if it's the right way to implement it.

Thank you !

Rather than mixing white and black list, redefine black list for certain virtual 
hosts.

Users may use multiple php_admin_flag/value for the same setting to override 
previous definitions in server contexts.

According to the tests I did, "php_admin_value disable_functions"
does not overwrite / redefine the global list, but complete it.

I think that having the ability to declare white lists
would make functions management easier.

For example, let's assume we have excluded 40 functions globally !
We need the exec function in one of our virtualhosts.
Instead of having to re-declare 39 functions in the virtualhost
(which does not seem to work), we would simply have to make declare :
"php_admin_value enable_functions exec"

Easier to manage, especially for envs with several / many virualhosts.
And so it would make the functions' management more secure.

Thank you very much !

In addition to the fact that "disable_functions" in a virtualhost
context does not semm to overwrite global setting but to complete it,
functions which have been disabled in a virtualhost context still work.

For example, phpinfo() shows that syslog function is disabled
(according to specific virtualhost setting
"php_admin_value disable_functions syslog"),
but syslog can still be used in PHP code.

See summary bug #65386