Generate a hash in the app and send it to the script with your other data. In the script, generate the same hash and compare if both hashes match. If they don't match, don't execute the rest of the script.
CBHash plugin (link to C3 version on the last page):
Make sure to use the same hashing function in the php, if you don't know how, try searching on stackoverflow.
You want to send player score to the server. In the app generate a hash MD5(secret_key & score)
Send to the PHP two values: score, hash.
In the PHP script, also generate a hash MD5(secret_key & score_received)
Compare if generated_hash is the same as received_hash.
Of course, the secret_key should be same in your app and in the PHP script, don't share it anywhere and don't send it with the data.