diff --git a/lisp/php-mode.el b/lisp/php-mode.el
index 9fc2b7b7..822a455a 100644
--- a/lisp/php-mode.el
+++ b/lisp/php-mode.el
@@ -458,7 +458,7 @@ PHP does not have an C-like \"enum\" keyword."
   php (append (c-lang-const c-class-decl-kwds) '("function")))
 
 (c-lang-defconst c-modifier-kwds
-  php '("abstract" "const" "final" "static" "case"))
+  php '("abstract" "const" "final" "static" "case" "readonly"))
 
 (c-lang-defconst c-protection-kwds
   "Access protection label keywords in classes."
diff --git a/tests/8.1/readonly.php b/tests/8.1/readonly.php
new file mode 100644
index 00000000..b7487243
--- /dev/null
+++ b/tests/8.1/readonly.php
@@ -0,0 +1,20 @@
+<?php
+
+class Foo
+{
+    private readonly string $p1;
+    public readonly int $p2;
+
+    public function __construct(
+        private readonly string $a1,
+        public readonly int $a2,
+    ) {}
+}
+
+__halt_compiler();
+
+claas Err
+{
+    /** Readonly property must have type */
+    public readonly $e1;
+}
diff --git a/tests/8.1/readonly.php.faces b/tests/8.1/readonly.php.faces
new file mode 100644
index 00000000..d5526835
--- /dev/null
+++ b/tests/8.1/readonly.php.faces
@@ -0,0 +1,84 @@
+;; -*- mode: emacs-lisp -*-
+(("<?php" . php-php-tag)
+ ("
+
+")
+ ("class" . php-class-declaration)
+ (" ")
+ ("Foo" . font-lock-type-face)
+ ("
+{
+    ")
+ ("private" . php-keyword)
+ (" ")
+ ("readonly" . php-keyword)
+ (" ")
+ ("string" . font-lock-type-face)
+ (" ")
+ ("$" . php-variable-sigil)
+ ("p1" . php-variable-name)
+ (";
+    ")
+ ("public" . php-keyword)
+ (" ")
+ ("readonly" . php-keyword)
+ (" ")
+ ("int" . font-lock-type-face)
+ (" ")
+ ("$" . php-variable-sigil)
+ ("p2" . php-variable-name)
+ (";
+
+    ")
+ ("public" . php-keyword)
+ (" ")
+ ("function" . php-keyword)
+ (" ")
+ ("__construct" . php-function-name)
+ ("(
+        ")
+ ("private" . php-keyword)
+ (" ")
+ ("readonly" . php-keyword)
+ (" ")
+ ("string" . font-lock-type-face)
+ (" ")
+ ("$" . php-variable-sigil)
+ ("a1" . php-variable-name)
+ (",
+        ")
+ ("public" . php-keyword)
+ (" ")
+ ("readonly" . php-keyword)
+ (" ")
+ ("int" . font-lock-type-face)
+ (" ")
+ ("$" . php-variable-sigil)
+ ("a2" . php-variable-name)
+ (",
+    ) {}
+}
+
+")
+ ("__halt_compiler" . php-keyword)
+ ("();
+
+")
+ ("claas" . font-lock-type-face)
+ (" ")
+ ("Err" . font-lock-variable-name-face)
+ ("
+{
+    ")
+ ("/** Readonly property must have type */" . font-lock-doc-face)
+ ("
+    ")
+ ("public" . php-keyword)
+ (" ")
+ ("readonly" . php-class)
+ (" ")
+ ("$" . php-variable-sigil)
+ ("e1" . php-variable-name)
+ (";
+}
+"))
diff --git a/tests/php-mode-test.el b/tests/php-mode-test.el
index 2b6185b7..f31b8daa 100644
--- a/tests/php-mode-test.el
+++ b/tests/php-mode-test.el
@@ -659,7 +659,8 @@ Meant for `php-mode-test-issue-503'."
 
 (ert-deftest php-mode-test-php81 ()
   "Test highlighting language constructs added in PHP 8.1."
-  (with-php-mode-test ("8.1/enum.php" :faces t)))
+  (with-php-mode-test ("8.1/enum.php" :faces t))
+  (with-php-mode-test ("8.1/readonly.php" :faces t)))
 
 (ert-deftest php-mode-test-lang ()
   "Test highlighting for language constructs."