What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED INBOUND INTO PHL. THE FLIGHT DECK DOOR WAS LOCKED AND FLIGHT CREW COULD NOT UNLOCK THE DOOR DURING CRUISE RESULTING IN THE FIRST OFFICER NOT BEING ABLE TO GET INTO THE FLIGHT DECK. THE FLIGHT CREW REPORTED THAT THE DOOR DID OPEN AT 8,000 FEET AND THE FIRRST OFFICER WAS ABLE TO GET IN THE FLIGHT DECK FOR LANDING. THE AIRCRAFT LANDED SAFELY AT PHL AND TAXIED TO THE GATE. MAINTENANCE FOUND THE FLIGHT DECK DOOR BOTTOM LATCH SEIZED / HARD TO MOVE AND REPLACED IT PER AMM 52-51-21. A SATISFACTORY OPERATIONAL CHECK WAS ACCOMPLISHED WITH NO ISSUES NOTED.