PHP equivalent of .net AES encryption

Question

I am working on a data exchange integration with my client and the data they send me is encrypted using their C# encrypt method (below).

My app is running

Answer 1

It seems like the PHP Script is Using the wrong Mode:
https://msdn.microsoft.com/en-us/library/system.security.cryptography.symmetricalgorithm.mode%28v=vs.110%29.aspx
The C# Functions do not set any Mode so the default is CBC.
The PHP part uses ECB instead, which is not only wrong, but insecure.

Answer 2

Can anyone spot the mistake?

Yes, and the big one isn't really your fault: mcrypt's confusing API strikes again.

That said, there are actually multiple mistakes here.

return rtrim( // unnecessary
    base64_encode(
        mcrypt_encrypt(
            MCRYPT_RIJNDAEL_256, // Not AES
            $secretKey, $plainText,
            MCRYPT_MODE_ECB, // BAD, use MCRYPT_MODE_CBC or 'ctr' instead
            mcrypt_create_iv(
                mcrypt_get_iv_size(      // unless you're going make this
                    MCRYPT_RIJNDAEL_256, // configurable, you should just
                    MCRYPT_MODE_ECB      // hard-code this as an integer
                ),
                MCRYPT_RAND) // BAD, use MCRYPT_DEV_URANDOM
        )
    ), "\0"
); 

If you're going to generate an IV, it should be communicated so your recipient can decrypt the same first block successfully. The C# code does this, the PHP does not.

From a cryptography engineering perspective, you should consider, both in C# land and in PHP, deploying an Encrypt then Authenticate protocol. See this blog post on encryption and authentication. Also, all the crypto code you've ever written is probably broken.