diff --git a/lib/CompilerVersion.php b/lib/CompilerVersion.php
index 405622de198..5bb285a988b 100644
--- a/lib/CompilerVersion.php
+++ b/lib/CompilerVersion.php
@@ -1318,14 +1318,18 @@ public static function supportsReadonlyBuiltin(): bool
     }
 
     /**
-     * readonly() visible to function_exists() — stable runtime only (#16357, re-#16292).
+     * readonly() visible to function_exists() — stable runtime or forward 8.4+ profile (#16357, #17693).
      *
-     * Callable under forward profile via {@see supportsReadonlyBuiltin()}; withheld from introspection on 8.4.0-dev
-     * reference harness like Zend 8.2.
+     * Withheld on 8.4.0-dev reference profile (matches Zend 8.2 phantom gate). Callable and advertised when
+     * {@see supportsReadonlyBuiltin()} is true (stable 8.4.0+ or explicit {@code PHP_COMPILER_PROFILE=8.4}).
      */
     public static function advertisesReadonlyBuiltin(): bool
     {
-        return version_compare(self::VERSION, '8.4.0', '>=');
+        if (version_compare(self::VERSION, '8.4.0', '>=')) {
+            return true;
+        }
+
+        return version_compare(self::languageProfileVersion(), '8.4.0', '>=');
     }
 
     /**
diff --git a/test/unit/ForwardProfilePhantomIntrospectionTest.php b/test/unit/ForwardProfilePhantomIntrospectionTest.php
index 3e7e616721f..27eb12eb480 100644
--- a/test/unit/ForwardProfilePhantomIntrospectionTest.php
+++ b/test/unit/ForwardProfilePhantomIntrospectionTest.php
@@ -270,19 +270,19 @@ public function testZendThreadIdAdvertisedOnForwardProfile84(): void
         }
     }
 
-    public function testReadonlyCallableButNotAdvertisedOnForwardProfile(): void
+    public function testReadonlyAdvertisedOnForwardProfile84(): void
     {
         $prev = getenv('PHP_COMPILER_PROFILE');
         putenv('PHP_COMPILER_PROFILE=8.4');
         try {
             $this->assertTrue(CompilerVersion::supportsReadonlyBuiltin());
-            $this->assertFalse(CompilerVersion::advertisesReadonlyBuiltin());
-            $this->assertFalse(BuiltinIntrospectionPolicy::functionIsAdvertised('readonly'));
+            $this->assertTrue(CompilerVersion::advertisesReadonlyBuiltin());
+            $this->assertTrue(BuiltinIntrospectionPolicy::functionIsAdvertised('readonly'));
 
             $runtime = new Runtime();
             $ctx = $runtime->vmContext;
             $this->assertTrue(isset($ctx->functions['readonly']));
-            $this->assertFalse(
+            $this->assertTrue(
                 \PHPCompiler\ext\standard\VmReflection::functionExists($ctx, 'readonly')
             );
         } finally {