Mysql connection from within php

[What you get is Not what you see]

Freshly installed on openbsd 4.6 mysql,php and php5-mysql packages.
Done the configs. Now php and mysql works. But I couldnt make it
connect to mysql from within php with such a command
mysql_connect("localhost","user","pass")
It used to give "Cant connect to mysql through socket error" till I
change the command to
mysql_connect(127.0.0.1,"user","pass")
I want to learn why?