So this is happening for exactly the same reason as https://bugs.php.net/bug.php?id=71917 (we don't try to set key length for variable length ciphers if the key is shorter than the default key size for the algorithm).

Unfortunately changing that as default would have exactly the same implication in terms of BC. It would stop working for people using shorter keys and relying on the fact that is filled with \0. So I would be inclined to introduce a new option for that which could possibly go to 7.1 if RM is fine with that or master (7.2) otherwise. I will create a PR.

A new constant OPENSSL_DONT_ZERO_PAD_KEY has been introduced to address that and it will be part of 7.1.8.

For example it can be used like:

openssl_encrypt("this is a test string","bf-ecb","12345678", OPENSSL_RAW_DATA | OPENSSL_DONT_ZERO_PAD_KEY));

Is there any way to have this fix backported to 7.0?

Since mcrypt was deprecated in 7.1, it makes it more difficult to transition to OpenSSL given this issue.

Use this function for OpenSSL to get the same key of mcrypt's blowfish:

function make_openssl_blowfish_key($key)
{
    if("$key" === '')
        return $key;

    $len = (16+2) * 4;
    while(strlen($key) < $len) {
        $key .= $key;
    }
    $key = substr($key, 0, $len);
    return $key;
}