function get_nested_projects($Projects, $ParentID = 0, $indent='')
     {
            $Ret = array();
            foreach($Projects as $Project)
            {
        echo '<pre>';print_r($Project);echo '<pre>';

        if(!isset($Project->parent_id))
        {
          echo 'project parent id';
        }

            }
     }

Output:

Array
(
    [0] => stdClass Object
        (
            [p_id] => 1
            [c_id] => 1
            [project_name] => Root Project
            [project_desc] => Root Project
            [date_added] => 2012-01-01 00:00:00
            [parent_id] => 0
            [allocated_hrs] => 100
            [date_from] => 2012-01-01 00:00:00
            [date_to] => 2012-01-01 00:00:00
            [status] => active
            [priority] => 1
            [completed] => 1
            [author] => 1
            [p_date_added] => 1st Jan 2012
            [date_from_formatted] => 2012-01-01
            [date_to_formatted] => 2012-01-01
            [proj_id] => 1
            [cust_name] => Root Customer
            [cust_id] => 1
            [parent_name] =>
        )

    [1] => stdClass Object
        (
            [p_id] => 120
            [c_id] => 2
            [project_name] => Another project
            [project_desc] => another project
            [date_added] => 2012-01-01 00:00:00
            [parent_id] => 2
            [allocated_hrs] => 100
            [date_from] => 2012-01-01 00:00:00
            [date_to] => 2012-01-01 00:00:00
            [status] => active
            [priority] => 1
            [completed] => 1
            [author] => 271
            [p_date_added] => 1st Jan 2012
            [date_from_formatted] => 2012-01-01
            [date_to_formatted] => 2012-01-01
            [proj_id] => 120
            [cust_name] => Customer 2
            [cust_id] => 2
            [parent_name] => Mini project
        )

    [2] => stdClass Object
        (
            [p_id] => 3
            [c_id] => 2
            [project_name] => This is a project
            [project_desc] => This is a projectThis is a projectThis is a projectThis is a projectThis is a projectThis is a projectThis is a projectThis is a projectThis is a project
            [date_added] => 2012-03-28 09:27:16
            [parent_id] => 1
            [allocated_hrs] => 100
            [date_from] => 2012-10-10 00:00:00
            [date_to] => 2012-11-10 00:00:00
            [status] => active
            [priority] => 1
            [completed] => 0
            [author] => 271
            [p_date_added] => 28th Mar 2012
            [date_from_formatted] => 2012-10-10
            [date_to_formatted] => 2012-11-10
            [proj_id] => 3
            [cust_name] => Customer 2
            [cust_id] => 2
            [parent_name] => Root Project
        )

)

project parent id