/tmp/mysql.sock issue

[Oliver Schulze L.]

Hi people,
I'm having this strange problem with: /tmp/mysql.sock

I have:
RH 7.2
IMP 3.0/Horde 2.0/Pear 4.1.0
PHP 4.0.6

If I configure horde to use use MySQL and host=localhost, I get this error:
A fatal error has occurred object(db_error)(7) {
 ["error_message_prefix"]=>
 string(0) ""
 ["mode"]=>
 int(1)
 ["level"]=>
 int(1024)
 ["code"]=>
 int(-24)
 ["message"]=>
 string(24) "DB Error: connect failed"
 ["userinfo"]=>
 string(95) " [nativecode=Can't connect to local MySQL server through 
socket '/tmp/mysql.sock' (2)] ** Array"
 ["callback"]=>
 NULL
}
[/var/www/html/mail/horde/lib/Prefs/sql.php : 102]

and can connect with:
$ mysql -h localhost -D horde -u horde -p

But if I configure Horde to use my.host.com as MySQL host with the same 
user/passwd,
Horde works and also this works:
$ mysql -h my.host.com -D horde -u horde -p

I don't know why PHP choose to comunicate with MySQL via this
file socket that does not exist:
/tmp/mysql.sock
and not via this file socket that does exist:
/var/lib/mysql/mysql.sock

In /etc/my.cnf I have:
[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock

If this is a problem with the PHP build by RedHat,
I will just add and entry in bugzilla.redhat.com but the problem
is that I don't know where the problem is.

How can I be sure where the problem is? MySQL, PHP, Pear?

Many Thanks
Oliver

-- 
Oliver Schulze L.
oliver@samera.com.py
Asuncion-Paraguay