Description:
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"chacha20-poly1305" is listed in the result of openssl_get_cipher_methods(). Wikipedia says it is an AEAD. I.e., if I call openssl_encrypt and provide a reference to a variable that is supposed to get the authentication tag, I expect it to get the tag. Instead, I get this warning:

PHP Warning:  openssl_encrypt(): The authenticated tag cannot be provided for cipher that doesn not support AEAD in /home/aep/aead-test/index.php on line 17

There is a pure PHP implementation of chacha20-poly1305, and it does support the AEAD mode (i.e. gets a tag). It is available as "leigh/aead-chacha20-poly1305" via composer, and the test script below uses it for reference.

Test script:
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<?php

# make sure that you used Composer to get "leigh/aead-chacha20-poly1305"
require __DIR__ . '/vendor/autoload.php';

$key = str_repeat('k', 32);
$nonce = str_repeat('n', 12);

$plaintext = '12345678';
$aad = '';
list($ciphertext, $tag) = \ChaCha20Poly1305\encrypt($key, $nonce, $aad, $plaintext);
$cl = implode(unpack("H*", $ciphertext));
$tl = implode(unpack("H*", $tag));

print("Pure PHP\n");
print("Ciphertext: $cl, tag: $tl\n");

$openssl_ct = openssl_encrypt($plaintext, "chacha20-poly1305", $key, OPENSSL_RAW_DATA, $nonce, $openssl_tag, $aad);
$ocl = implode(unpack("H*", $openssl_ct));
$otl = implode(unpack("H*", $openssl_tag));

print("OpenSSL\n");
print("Ciphertext: $ocl, tag: $otl\n");


Expected result:
----------------
Pure PHP
Ciphertext: bfcd0d66f7e49c21, tag: e48864c53c4deb83bcee96add54ef851
OpenSSL
Ciphertext: bfcd0d66f7e49c21, tag: e48864c53c4deb83bcee96add54ef851


Actual result:
--------------
Pure PHP
Ciphertext: bfcd0d66f7e49c21, tag: e48864c53c4deb83bcee96add54ef851
PHP Warning:  openssl_encrypt(): The authenticated tag cannot be provided for cipher that doesn not support AEAD in /home/aep/aead-test/index.php on line 17
OpenSSL
Ciphertext: bfcd0d66f7e49c21, tag: