[SOLVED] Integrating PHP, SSH and ssh-agent

I plan to write a PHP script that makes an SSH connection. I've investigated how to do this and this looks the most promising solution: https://github.com/phpseclib/phpseclib My only issue is how to handle the fact that my SSH key has a passphrase, and I don't want to have to enter it every time I run the script. For every day SSH use I have ssh-agent running in the background, and it's configured to use pinentry. This makes it so that I don't have to enter my passphrase EVERY time. Any ideas as to how I could get PHP and ssh-agent to talk to each other? My only clue is that ssh-agent sets an environment variable, SSH_AUTH_SOCK, pointing to a socket file.

While the documentation for phpseclib addresses this issue, its answer is silly (just put the passphrase in the code): http://phpseclib.sourceforge.net/ssh/2.0/auth.html#encrsakey

UPDATE: I've looked more into phpseclib and written my own simple wrapper class. However, I cannot get it to login either through ssh-agent or by supplying my RSA key. Only password-based authentication works, contrary to my experiences logging in directly with the ssh command. Here is my code:

<?php
// src/Connection.php
declare(strict_types=1);
namespace MyNamespace\PhpSsh;

use phpseclib\System\SSH\Agent;
use phpseclib\Net\SSH2;
use phpseclib\Crypt\RSA;
use Exception;

class Connection
{
    private SSH2 $client;
    private string $host;
    private int $port;
    private string $username;

    /**
     * @param string $host
     * @param int $port
     * @param string $username
     * 
     * @return void
     */
    public function __construct(string $host, int $port,
        string $username)
    {
        $this->host = $host;
        $this->port = $port;
        $this->username = $username;
        $this->client = new SSH2($host, $port);
    }

    /**
     * @return bool
     */
    public function connectUsingAgent(): bool
    {
        $agent = new Agent();
        $agent->startSSHForwarding($this->client);
        return $this->client->login($this->username, $agent);
    }

    /**
     * @param string $key_path
     * @param string $passphrase
     * 
     * @return bool
     * @throws Exception
     */
    public function connectUsingKey(string $key_path, string $passphrase = ''): bool
    {
        if (!file_exists($key_path)) {
            throw new Exception(sprintf('Key file does not exist: %1$s', $key_path));
        }

        if (is_dir($key_path)) {
            throw new Exception(sprintf('Key path is a directory: %1$s', $key_path));
        }

        if (!is_readable($key_path)) {
            throw new Exception(sprintf('Key file is not readable: %1$s', $key_path));
        }

        $key = new RSA();

        if ($passphrase) {
            $key->setPassword($passphrase);
        }

        $key->loadKey(file_get_contents($key_path));

        return $this->client->login($this->username, $key);
    }

    /**
     * @param string $password
     * 
     * @return bool
     */
    public function connectUsingPassword(string $password): bool
    {
        return $this->client->login($this->username, $password);
    }

    /**
     * @return void
     */
    public function disconnect(): void
    {
        $this->client->disconnect();
    }

    /**
     * @param string $command
     * @param callable $callback
     * 
     * @return string|false
     */
    public function exec(string $command, callable $callback = null)
    {
        return $this->client->exec($command, $callback);
    }

    /**
     * @return string[]
     */
    public function getErrors(): array {
        return $this->client->getErrors();
    }
}

And:

<?php
    // test.php
    use MyNamespace\PhpSsh\Connection;

    require_once(__DIR__ . '/vendor/autoload.php');

    (function() {
        $host = '0.0.0.0'; // Fake, obviously
        $username = 'user'; // Fake, obviously
        $connection = new Connection($host, 22, $username);
        $connection_method = 'AGENT'; // or 'KEY', or 'PASSWORD'

        switch($connection_method) {
            case 'AGENT':
                $connected = $connection->connectUsingAgent();
                break;
            case 'KEY':
                $key_path = getenv( 'HOME' ) . '/.ssh/id_rsa.pub';
                $passphrase = trim(fgets(STDIN)); // Pass this in on command line via < key_passphrase.txt
                $connected = $connection->connectUsingKey($key_path, $passphrase);
                break;
            case 'PASSWORD':
            default:
                $password = trim(fgets(STDIN)); // Pass this in on command line via < password.txt
                $connected = $connection->connectUsingPassword($password);
                break;
        }

        if (!$connected) {
            fwrite(STDERR, "Failed to connect to server!" . PHP_EOL);
            $errors = implode(PHP_EOL, $connection->getErrors());
            fwrite(STDERR, $errors . PHP_EOL);
            exit(1);
        }

        $command = 'whoami';
        $result = $connection->exec($command);
        echo sprintf('Output of command "%1$s:"', $command) . PHP_EOL;
        echo $result . PHP_EOL;

        $command = 'pwd';
        $result = $connection->exec($command);
        echo sprintf('Output of command "%1$s:"', $command) . PHP_EOL;
        echo $result . PHP_EOL;

        $connection->disconnect();
    })();

The SSH2 class has a getErrors() method, unfortunately it wasn't logging any in my case. I had to debug the class. I found that, whether using ssh-agent or passing in my key, it always reached this spot (https://github.com/phpseclib/phpseclib/blob/2.0.23/phpseclib/Net/SSH2.php#L2624):

<?php
// vendor/phpseclib/phpseclib/phpseclib/Net/SSH2.php: line 2624
extract(unpack('Ctype', $this->_string_shift($response, 1)));

switch ($type) {
    case NET_SSH2_MSG_USERAUTH_FAILURE:
        // either the login is bad or the server employs multi-factor authentication
        return false;
    case NET_SSH2_MSG_USERAUTH_SUCCESS:
        $this->bitmap |= self::MASK_LOGIN;
        return true;
}

Obviously the response being returned is of type NET_SSH2_MSG_USERAUTH_FAILURE. There's nothing wrong with the login, of that I'm sure, so per the comment in the code that means the host (Digital Ocean) must use multi-factor authentication. Here's where I'm stumped. What other means of authentication am I lacking? This is where my understanding of SSH fails me.