Hrrm, I now get a "Segmentation fault (core dumped)" when I try to require a private package with composer.

Looks like the steps above are roughly the same as sudo pecl install https://pecl.php.net/get/ssh2-1.2.tgz.

Regarding the segfault error, looks the same as the bug reported here: https://bugs.php.net/bug.php?id=79631. I'm able to connect, and read dir contents, but get a segfault error when trying to close the resource.

<?php

$ctx = stream_context_create([
    'ssh2' => ['pubkey_file' => '/home/user/id_rsa.pub', 'privkey_file' => '/home/user/.ssh/id_rsa']
]);
$dirhandle = opendir("ssh2.sftp://user@host.example:22/path/", $ctx);

var_dump(readdir($dirhandle)); // => string(1) "."

closedir($dirhandle); // => "Segmentation fault"

This script also returns a segfault error when using incorrect key files.

As reported in https://bugs.php.net/bug.php?id=79631, this works:

<?php

$con = ssh2_connect('host.example', 22);
$auth = ssh2_auth_pubkey_file($con, 'user', '/home/user/.ssh/id_rsa.pub', '/home/user/.ssh/id_rsa');
$sftp = ssh2_sftp($con);

$handle = opendir('ssh2.sftp://'.intval($sftp).'/domains/');

var_dump(closedir($handle));
// => NULL

var_dump(ssh2_disconnect($sftp)); // when this step is skipped, the skript ends with "Segmentation fault"
// => bool(true)

var_dump(ssh2_disconnect($con));
// => bool(true)

So probably composer fails because it tries to read directory contents with the stream wrapper.