wordpress.php

 <?php
    // get private key
    $key = get_user_meta( 1, 'magic_sig_private_key', true );

		$date = gmdate('D, d M Y H:i:s T');
		$signed_string = "(request-target): post /inbox\nhost: mastodon.social\ndate: $date";
		$signature = null;
		openssl_sign( $signed_string, $signature, $key, OPENSSL_ALGO_SHA256 );

		$signature = 'keyId="https://pfefferle.org/author/pfefferle/#main-key",algorithm="rsa-sha256",headers="(request-target) host date",signature="' . base64_encode( $signature ) . '"';

		$response = wp_safe_remote_post( 'https://mastodon.social/inbox', array(
			'headers' => array(
				'date' => $date,
				'content-type' => 'application/activity+json',
				'signature' => $signature,
			),
			'body' => '{"@context": "https://www.w3.org/ns/activitystreams","id": "https://pfefferle.org/1538041738","type": "Accept","actor": "https://pfefferle.org/author/pfefferle/","object": "https://mastodon.social/users/pfefferle"}',
		) );