mySQL 4.0.20 -- Can't connect to local MySQL server through socket '/tmp/mysql.sock' (13)

[Wells Wang]

i installed mysql 4.0.20 on a redhat enterprise advanced server 3 box,
i can startup mysqld without any error message,
and i can successfully use "mysql -u root -p" to connect into database at localhost.
but i cannot use php to connect mysql.

my socket file is /tmp/mysql.sock

at localhost, i can use mysql to connect database successfully.
but use php, i get this error message:
Can't connect to local MySQL server through socket '/tmp/mysql.sock' (13)

my /tmp/mysql.sock is for user mysql and group mysql. and it's attribute is 777.

apache is apache 2.0.46
php is php 4.3.8

in php.ini , i've set default socket into '/tmp/mysql.sock'.

i cannot solve this problem by myself,
anyone can help me?