php.ini causes disk thrashing in mysql

[Marin]

Hello. For a while I was developing a site with Apache + PHP + MySQL
and, because I hadn't put the php.ini in the right place, php was just
using its default settings. During this time mysql was running fine.
Recently I put the php.ini file in the right place and since then
mysql thrashes the disk intensely during a query, even for really
simple SELECT queries. A php webpage that should finish in less than 2
seconds now takes 10 seconds. I tried duplicating php's default
settings (for mysql) in the php.ini file and it still happens, which
suggests this problem is caused by a setting in the php.ini file which
is not [obviously] related to mysql. If I can't fix this then I'll
have to run php without an php.ini file, which is stupid. Anyone have
any ideas about what settings are causing the disk thrashing?

Many thanks,
Marin