LAX -> ShangHai -> Phnom Penh

Hi all,

I plan a trip to Cambodia with stop over in Shanghai, but the overlay is too short 13 hours. So I plan to buy a ticket from LAX to Shanghai (stay for 3 days) then from Shanghai to Phnom Penh on a seperate purchase but I will buy both ticket round trip ahead of time.. is this still valid for a 144 hours free entry?