diff -r be944660c56a -r 3d72ae0968f4 wp/wp-includes/php-compat/readonly.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-includes/php-compat/readonly.php	Tue Sep 27 16:37:53 2022 +0200
@@ -0,0 +1,37 @@
+<?php
+/**
+ * Conditionally declares a `readonly()` function, which was renamed
+ * to `wp_readonly()` in WordPress 5.9.0.
+ *
+ * In order to avoid PHP parser errors, this function was extracted
+ * to this separate file and is only included conditionally on PHP 8.1.
+ *
+ * Including this file on PHP >= 8.1 results in a fatal error.
+ *
+ * @package WordPress
+ * @since 5.9.0
+ */
+
+/**
+ * Outputs the HTML readonly attribute.
+ *
+ * Compares the first two arguments and if identical marks as readonly.
+ *
+ * This function is deprecated, and cannot be used on PHP >= 8.1.
+ *
+ * @since 4.9.0
+ * @deprecated 5.9.0 Use wp_readonly() introduced in 5.9.0.
+ *
+ * @see wp_readonly()
+ *
+ * @param mixed $readonly One of the values to compare.
+ * @param mixed $current  Optional. The other value to compare if not just true.
+ *                        Default true.
+ * @param bool  $echo     Optional. Whether to echo or just return the string.
+ *                        Default true.
+ * @return string HTML attribute or empty string.
+ */
+function readonly( $readonly, $current = true, $echo = true ) {
+	_deprecated_function( __FUNCTION__, '5.9.0', 'wp_readonly()' );
+	return wp_readonly( $readonly, $current, $echo );
+}