Usage of 'hostkey' option makes sence when you need to check authenticity of the remote server, e.g. by means of ssh2_fingerprint function. For that purpose you specify a list of possible algorithms sorted by priority in descending order. Supported values also include 'ssh-ed25519', 'ecdsa-sha2-nistp256', 'ecdsa-sha2-nistp384' and 'ecdsa-sha2-nistp521'.
For instance:
<?php
$ssh = ssh2_connect('192.168.3.1', 22, ['hostkey' => 'ssh-ed25519,ssh-rsa']);
$md5 = ssh2_fingerprint($ssh); ?>
Note, if the remote server does not have a key of appropriate type connection will fail:
<?php
$ssh = ssh2_connect('192.168.3.1', 22, ['hostkey' => 'ssh-dss']);
PHP Warning: ssh2_connect(): Error starting up SSH connection(-5): Unable to exchange encryption keys in php shell code on line 1
PHP Warning: ssh2_connect(): Unable to connect to 192.168.3.1 in php shell code on line 1
?>
