请教：这段 PHP 代码中，$publickey = $publickey["key"]; 这一行是什么意思？

<?php


if (isset($_SERVER['HTTPS']) )

{
echo "SECURE: This page is being accessed through a secure connection.";
}

else

{
echo "UNSECURE: This page is being access through an unsecure connection.";
}

$cOnfig= array(

"digest_alg" => "sha512",

"private_key_bits" => 385,

"private_key_type" => OPENSSL_KEYTYPE_RSA,

);

// Create the keypair

$res=openssl_pkey_new($config);

// Get private key

openssl_pkey_export($res, $privatekey);

// Get public key

$publickey=openssl_pkey_get_details($res);

$publickey= $publickey["key"];

echo "Private Key:<BR>$privatekey

Public Key:<BR>$publickey<BR><BR>";

$cleartext = '1234 567gongji 珍珠港虎虎虎';

echo "Clear text:
$cleartext<BR><BR>";

openssl_public_encrypt($cleartext, $crypttext, $publickey);

echo "Crypt text:$crypttext<BR><BR>";

openssl_private_decrypt($crypttext, $decrypted, $privatekey);

echo "Decrypted text:<BR>$decrypted<br><br>";

?>

["key"] 这个能理解成 php 保留的数组值吗？