PHP - Class Design Pattern Hellp

I'm writing a timesheet application which would be used by employees and approved/rejected by supervisors. A timesheet has various permissions and only people with certain roles can do certain actions (like for example only a supervisor can approve a timesheet and only if it belongs to an employee under the supervisor's list of employees).

I'm trying to figure out how to structure my list of business objects (models). There are various validations I need to do before I can invoke methods like GetTimesheet or SaveTimesheet or ApproveTimesheet. See the following psuedo-code for an understanding of how my app should work:

GetTimesheetByUserIdAndMonth:

    -Validate parameters like UserId, Month-Year value

    -Check Permissions:
    -   If Logged In User is A Supervisor, then see if Timesheet belongs to either this user or to a user under Supervisor's list of Employees
    -   Else, check if Timesheet belongs to logged in User


SaveTimesheet

    -Validate parameters like UserId, Month-Year value

    -Check Permissions (same as above)

    -Check if user has write ability for timesheet (for example if user had already submitted timesheet before then he can't re-save or re-submit)

SubmitTimesheet

    -See if user has a supervisor assigned

ApproveTimesheet, RejectTimesheet

    -Logged in User must be a Supervisor, otherwise throw an error

I'm thinking I would need a MyAccount class for the person who is logged in, a User class to represent the person who the timesheet belongs to, a Timesheet class, and maybe some sort of Validation class.

Can someone tell me how best to architect this code and what sort of classes and validation methods I should have? I already have this code working in 100% procedural code...it is very difficult to read and maintain. I'm not looking for full implementation details, just an overall class achitecture/structure. Please give me some ideas and provide me some psuedo-code how to accomplish the above tasks. I can provide more details if necessary.

Thanks in advance.