Common way to store db-password of open session?

Hi, is threre a most advisble way to store db-passwords of an open user-session? As far as I get it, a common login strategy is to let the user login by name&password, check it, store a login=TRUE as php-session variable and later use a common dbuser+pw to query data provided "login" is TRUE. This way one wouldn't have to store the users pw or actually the user wouldn't have a real db-account but rather an application account. Is this really better or equal than using real db-accounts? Should I rather store the db-credentials in a session or cookies? Session is vulnerable as any host-user could look into /tmp. This would generally be a trusted few though. On the other hand cookies could be manipulated by the user or at least be spied upon on the way between user and web-host everytime the credentials are needed for a query.