there is no easy or guaranteed way...
using SSL helps make it harder for people to intercept requests. You can calculate a hash that the PHP script can use to check if the score is legitimate. But it is not very hard for people to reverse engineer that, or use programs to change the score to whatever they want inside your game (making the request look real to your database). All you can really do is try to discourage causal hackers - the good ones will always find a way.
I have seen some pretty good suggestions on the forums over the years, but it might take some digging to find those posts now.
monitor your database, and remove the obvious fake scores...