Description:
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I'm testing the new features of PHP 7.1 for the support of AEAD using OpenSSL and I noticed that the openssl_error_string() returns an empty string in case of authentication error. It will be nice to have a message here to recognize this new case.

Test script:
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$iv = random_bytes(openssl_cipher_iv_length('aes-256-gcm'));
$key = random_bytes(32);
$data = random_bytes(1024);
$ciphertext = openssl_encrypt($data, 'aes-256-gcm', $key, OPENSSL_RAW_DATA, $iv, $tag);
// Change 1 bit in ciphertext
$i = rand(0, mb_strlen($ciphertext, '8bit') - 1);
$ciphertext[$i] = $ciphertext[$i] ^ chr(1);
$decrypt = openssl_decrypt($ciphertext, 'aes-256-gcm', $key, OPENSSL_RAW_DATA, $iv, $tag);
if (false === $decrypt) {
  printf("OpenSSL message: %s", openssl_error_string());
}

Expected result:
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OpenSSL message: authentication error

Actual result:
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OpenSSL message: