Subject: can not select sybase db?urgent

[wxdqz]

1the function is like that:
function connect_sybase()
{
global $server,$sqluser,$sqlpass,$sqldb;
$db_con = sybase_pconnect($server,$sqluser,$sqlpass) OR DIE("can not connect to database,script halted.contact sa!");;
@sybase_select_db("$sqldb",$db_con) or die("cannot select your db,script halted,pls contact sa");
return $db_con;
}

2when call this.
90percent time is ok,but sometimes it is display "can not select your db..".

3 i don't know why, need your help.