Zephir Returned values by functions can only be assigned to variant variables

Question

So, i want to make an PHP extension by Zephir that can encrypt and encode a source code, but i have an error which is :

Error Message:

[ERROR] Returned values by functions can only be assigned to variant variables in                 
         /home/ubuntu/ta/utils/utils/Cryptix.zep on line 15                                        
                                                                                                   
Script zephir handling the __exec_command event returned with error code 1

Here is the Code:

Cryptix.zep

namespace Utils;

class Cryptix
{
    /**
     * 
     *
     * @param string type
     * @param string file
     * 
     */

    public function encryptFile(type, file)
    {
        string code = file_get_contents(file, true);
        string enc_code = "<?php Utils::decode('"+type+"', '"+openssl_enc(type, code)+"'); ?>";
        file_put_contents(file + ".original", code);
        file_put_contents(file, enc_code);
    }

    public function openssl_enc(string method, string data)
    {
        string firstkey = "Lk5Uz3slx3BrAghS1aaW5AYgWZRV0tIX5eI0yPchFz4=";
        string secondkey = "EZ44mFi3TlAey1b2w4Y7lVDuqO+SRxGXsa7nctnr/JmMrA2vN6EJhrvdVZbxaQs5jpSe34X3ejFK/o9+Y5c83w==";

        string first_key = base64_decode(firstkey);
        string second_key = base64_decode(secondkey);

        string iv_length = openssl_cipher_iv_length(method);
        string iv = openssl_random_pseudo_bytes(iv_length);

        string first_encrypted = openssl_encrypt(data, method, first_key, OPENSSL_RAW_DATA, iv);
        string second_encrypted = hash_hmac("sha3-512", first_encrypted, second_key, true);

        string output = base64_encode(iv + second_encrypted + first_encrypted);
        return output;
    }
}

what should i do to fix this error? thank you so much for your help.