That is totally insecure, because you are using raw RSA without padding.
Your application needs a signature, so you should not be dealing with encryptions and decryptions. For instance, PKCS#1 v1.5 is a good protocol, even though the signature is a piece of data that must be appended to what you want to prove the authenticity of.
To verify a PKCS#1 v1.5 signature in Python, you do:
from Crypto.PublicKey import RSA
from Crypto.Signature import PKCS1_v1_5
from Crypto.Hash import SHA
rsa_key = RSA.importKey(open(verification_key_file, "rb").read())
verifier = PKCS1_v1_5.new(rsa_key)
h = SHA.new(data_to_verify)
if verifier.verify(h, signature_received_with_the_data):
print "OK"
else:
print "Invalid"
I would strongly recommend to change the PHP code so that it creates such a signature.