boost.php

<?php

add_action( 'activitypub_safe_remote_post_response', function( $response, $url, $body, $user_id ) {
	$activity  = json_decode( $body );

	if ( empty( $activity->type ) || 'Create' !== $activity->type ) {
		error_log( 'Not a Create activity.' );
		return;
	}

	$url     = $activity->object->id;
	$post_id = url_to_postid( $url );

	// error_log( $url );
	// error_log( $post_id );

	if ( '' !== get_post_meta( $post_id, '_share_on_mastodon_boosted', true ) ) {
		error_log( 'Already boosted.' );
		return;
	}

	if ( ! filter_var( $url, FILTER_VALIDATE_URL ) ) {
		error_log( 'Invalid URL.' );
		return;
	}

	if ( ! class_exists( '\\Share_On_Mastodon\\Share_On_Mastodon' ) ) {
		error_log( 'Share on Mastodon not active.' );
		return;
	}

	$options = \Share_On_Mastodon\get_options();

	if ( empty( $options['mastodon_host'] ) ) {
		error_log( 'Missing Mastodon instance.' );
		return;
	}

	if ( empty( $options['mastodon_access_token'] ) ) {
		error_log( 'Missing Mastodon token.' );
		return;
	}

	$args = array(
		'q'       => $url,
		'type'    => 'statuses',
		'resolve' => true,
	);

	// Ask our Mastodon instance to "search" for this URL. This should also add it to the instance's database, if it doesn't already exist.
	$response = wp_remote_get(
		esc_url_raw( $options['mastodon_host'] . '/api/v2/search' ) . '?' . http_build_query( $args ), // @todo: Use `add_query_arg()`?
		array(
			'headers' => array(
				'Authorization' => 'Bearer ' . $options['mastodon_access_token'],
			),
			'timeout' => 15,
		)
	);

	\Share_On_Mastodon\debug_log( $response );

	if ( is_wp_error( $response ) ) {
		// Something went wrong.
		\Share_On_Mastodon\debug_log( $response );
		update_post_meta( $post_id, '_share_on_mastodon_boosted', '0' ); // On failure.
		return;
	}

	$json = json_decode( $response['body'], true );

	if ( ! empty( $json['statuses'][0]['id'] ) && 1 === count( $json['statuses'] ) ) {
		// Our instance returned a single status. We need its ID in order to boost it.
		$response = wp_remote_post(
			esc_url_raw( $options['mastodon_host'] . "/api/v1/statuses/{$json['statuses'][0]['id']}/reblog" ),
			array(
				'headers' => array(
					'Authorization' => 'Bearer ' . $options['mastodon_access_token'],
				),
				'timeout' => 15,
			)
		);

		// @todo: Check the response code instead.
		if ( is_wp_error( $response ) ) {
			// Something went wrong.
			\Share_On_Mastodon\debug_log( $response );
			update_post_meta( $post_id, '_share_on_mastodon_boosted', '0' ); // On failure.
			return;
		} elseif ( ! empty( $response['body'] ) ) {
			\Share_On_Mastodon\debug_log( $response['body'] );
			update_post_meta( $post_id, '_share_on_mastodon_boosted', sanitize_text_field( $json['statuses'][0]['id'] ) ); // On success.
		}
	}

	update_post_meta( $post_id, '_share_on_mastodon_boosted', '0' ); // On failure.
}, 999, 4 );

Does not work if the app (the Share on Mastodon plugin) was registered without read:search scope! I.e., requires some manual hacking.