I have an object $rows
If I use echo $rows the content is displayed in my html page, however I would like to select only some parts of the content but I cannot see inside the variable how it is structured... thanks
Have you looked at var_dump and friends? var_dump() works purely through side effects, so it's great as a debug aid but useless if you want to do something else with the info it spits out. That's when you reach for var_export() and maybe print_r().
A similar question here on SO shows additional ways to inspect your object - get_object_vars() for the object's state, for instance.
Try
print_r($rows);
to see the structure, then use
$rows['property_name'];
to access single properties
Use var_dump/print_r or the XDebug extension to inspect your vars/objects.