PHP Encryption and Decryption with Data from POST Method using C#

The reason it doesn't work is your padding is wrong. PKCS7 is the byte value of the pad length repeated(i.e. 00000010 00000010 if your padding 2 bytes). It is not the string value "0202", It appears there aren't any php functions that do this correctly, so I'd sugest you use a a mode of operation that does not need padding. OFB is supported by both c# and php.

YOU CANNOT USE A Fixed IV. For cbc mode, its fairly insecure, for OFB, its completely insecure. Use mcrypt_create_iv to get a new random one each time. Then just prepend the IV to the ciphertext when you send it ( it does not need to be encrypted). As a note, one problem you may already have hit is that php uses a string and C# uses byts for the IV and you may not be getting the correct conversion even now . I'd probably use hex and the functions to covert to/from that just to be sure.

Second, you need to use something to detect when people tamper with your data, otherwise they potentially read the cipher text via error codes/ timing issues in the underlying crypto libraries. Hmacs work well and are supported here for php and here for c#. HMAC your IV+ciphertext message and prepend the output to it . On the other end, run the c# equivalent function over the same data, and then compare the HMAC values. If they are the same,you safe, if not, reject.