You can find ora_plogon info at
http://www.php.net/manual/en/function.ora-plogon.php
int ora_plogon (string user, string password)
If you configured PHP and Oracle properly, you just needed a valid Oracle username and password to connect.
$conn = ora_plogin( "myuser@TNSNAME", "mypasswd" );
But why do you not use OCI8 which is more flexible than the standard Oracle extension?