Compat: introduce a conditional to detect ActivityPub requests

This conditional could be used within the plugin, but also by third-party plugins, to detect whether a request is an ActivityPub request, without having to manually check for query vars and headers every time.

includes/functions.php

@@ -228,3 +228,47 @@ function is_tombstone( $wp_error ) {
 
 	return false;
 }
+
+/**
+ * Check if a request is for an ActivityPub request.
+ *
+ * @return bool False by default.
+ */
+function is_activitypub_request() {
+	global $wp_query;
+
+	/*
+	 * ActivityPub requests are currently only made for
+	 * author archives, singular posts, and the homepage.
+	 */
+	if ( ! \is_author() && ! \is_singular() && ! \is_home() ) {
+		return false;
+	}
+
+	// One can trigger an ActivityPub request by adding ?activitypub to the URL.
+	global $wp_query;
+	if ( isset( $wp_query->query_vars['activitypub'] ) ) {
+		return true;
+	}
+
+	/*
+	 * The other (more common) option to make an ActivityPub request
+	 * is to send an Accept header.
+	 */
+	if ( isset( $_SERVER['HTTP_ACCEPT'] ) ) {
+		$accept = $_SERVER['HTTP_ACCEPT'];
+
+		/*
+		 * $accept can be a single value, or a comma separated list of values.
+		 * We want to support both scenarios,
+		 * and return true when the header includes at least one of the following:
+		 * - application/activity+json
+		 * - application/ld+json
+		 */
+		if ( preg_match( '/(application\/(ld\+json|activity\+json))/', $accept ) ) {
+			return true;
+		}
+	}
+
+	return false;
+}

readme.txt

@@ -115,6 +115,7 @@ Project maintained on GitHub at [automattic/wordpress-activitypub](https://githu
 
 = Next =
 
+* Compatibility: add a new conditional, `\Activitypub\is_activitypub_request()`, to allow third-party plugins to detect ActivityPub requests.
 * Compatibility: add hooks to allow modifying images returned in ActivityPub requests.
 * Compatibility: indicate that the plugin is compatible and has been tested with the latest version of WordPress, 6.2.