Summary: changed memory_limit in master and local php.ini and yet no change in the local value for a particular virtual host.
Trying to improve performance, I set the memory_limit to 1024M in /etc/php.ini
phpinfo() shows Master and Local values for other virtual hosts on the server as 1024M. Changing the value in /etc/php.ini changes all values, except one.
One site is stuck with a local value of 256M.
I thought I found the problem: there is a php.ini file (which I didn't know about) in that site's root, and it had
memory_limit = 256M
I changed it to 1024M. Problem solved? No. And now I don't know where to look. Obviously, I've restarted apache (/etc/init.d/httpd restart), and that usually does the trick.
I also turned off APC cache, though I don't think it would cache ini files.
And finally, I tried adding this to the virtual host in httpd.conf:
php_value memory_limit 536870912
(yes, that would be 512MB) And that had no effect whatsoever.
What else could be the problem?