diff --git a/activitypub.php b/activitypub.php
index 38a3d07..99b4056 100644
--- a/activitypub.php
+++ b/activitypub.php
@@ -17,6 +17,7 @@ namespace Activitypub;
 
 use function Activitypub\site_supports_blocks;
 
+require_once __DIR__ . '/includes/compat.php';
 require_once __DIR__ . '/includes/functions.php';
 
 /**
diff --git a/includes/compat.php b/includes/compat.php
new file mode 100644
index 0000000..4bee640
--- /dev/null
+++ b/includes/compat.php
@@ -0,0 +1,37 @@
+<?php
+/**
+ * ActivityPub implementation for WordPress/PHP functions either missing from older WordPress/PHP versions or not included by default.
+ */
+
+if ( ! function_exists( 'str_starts_with' ) ) {
+	/**
+	 * Polyfill for `str_starts_with()` function added in PHP 8.0.
+	 *
+	 * Performs a case-sensitive check indicating if
+	 * the haystack begins with needle.
+	 *
+	 * @param string $haystack The string to search in.
+	 * @param string $needle   The substring to search for in the `$haystack`.
+	 * @return bool True if `$haystack` starts with `$needle`, otherwise false.
+	 */
+	function str_starts_with( $haystack, $needle ) {
+		if ( '' === $needle ) {
+			return true;
+		}
+
+		return 0 === strpos( $haystack, $needle );
+	}
+}
+
+if ( ! function_exists( 'get_self_link' ) ) {
+	/**
+	 * Returns the link for the currently displayed feed.
+	 *
+	 * @return string Correct link for the atom:self element.
+	 */
+	function get_self_link() {
+		$host = wp_parse_url( home_url() );
+		$path = isset( $_SERVER['REQUEST_URI'] ) ? sanitize_text_field( wp_unslash( $_SERVER['REQUEST_URI'] ) ) : '';
+		return esc_url( apply_filters( 'self_link', set_url_scheme( 'http://' . $host['host'] . $path ) ) );
+	}
+}
diff --git a/includes/functions.php b/includes/functions.php
index ffb3c60..df630d4 100644
--- a/includes/functions.php
+++ b/includes/functions.php
@@ -439,19 +439,6 @@ function is_single_user() {
 	return false;
 }
 
-if ( ! function_exists( 'get_self_link' ) ) {
-	/**
-	 * Returns the link for the currently displayed feed.
-	 *
-	 * @return string Correct link for the atom:self element.
-	 */
-	function get_self_link() {
-		$host = wp_parse_url( home_url() );
-		$path = isset( $_SERVER['REQUEST_URI'] ) ? sanitize_text_field( wp_unslash( $_SERVER['REQUEST_URI'] ) ) : '';
-		return esc_url( apply_filters( 'self_link', set_url_scheme( 'http://' . $host['host'] . $path ) ) );
-	}
-}
-
 /**
  * Check if a site supports the block editor.
  *