soselo/api/v1/network/resolve-instance/mod.php

<?php

$_ = function() {
	$hostname = trim($_GET['hostname']);

	$ch = curl_init();
	curl_setopt($ch, CURLOPT_URL, "https://$hostname/.well-known/host-meta");
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
	curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
	$output = curl_exec($ch);
	curl_close($ch);

	preg_match('/(?<=")[^"]+\/.well-known\/webfinger/', $output, $m);
	if (count($m) === 0) {
		return apiresult([
			'error' => 'Could not find /.well-known/webfinger URL in instance given XML'
		]);
	}
	return apiresult([
		'instance' => 'https://'.parse_url($m[0], PHP_URL_HOST),
	]);
};
$_();