Your configuration in the Mirth is directly to connect with OpenEMR’s database. No need to have PHP code level changes.You can create mirth channel to send/receive HL7 messages between source/destination.
OpenEMR is having options to create ORM messages.
Patient>New Encounter form>Administrative>Procedure Order
Refer here to know more about Procedure order messages.
In addition,you may also refer Mirth Connect’s user guide to know more.