fix(php): emit native backed-enum defaults as Type::CASE

AbstractPhpCodegen.toEnumDefaultValue used self::FQCN_CASE, which is invalid
for PHP 8.1 enums when model properties use enum $ref with a sibling default.
Emit datatype::caseName instead and document the upstream data flow.

Extend optional-enum-query-ref-default.yaml with a pet-store CreatePetRequest
example; add PhpSymfonyServerCodegenTest regression and refresh the issue draft.

Author: JerrySLau

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractPhpCodegen.java

@@ -781,9 +781,25 @@ public String toEnumValue(String value, String datatype) {
         }
     }
 
+    /**
+     * Builds the PHP expression for a backed enum case default (PHP 8.1+ {@code enum}).
+     * <p>
+     * The legacy {@code self::}{@code <datatype>_<CASE>} form came from class-constant style enums (#10273) and is
+     * invalid when {@code datatype} is a namespaced class: {@code self::} only resolves constants on the current
+     * class. Native enums must use {@code EnumType::CASE}.
+     * <p>
+     * Execution: {@code datatype} is produced upstream (e.g. {@link DefaultCodegen#updateCodegenPropertyEnum}) via
+     * {@link #getTypeDeclaration(Schema)} for the referenced enum schema; {@code value} is the sanitized case name
+     * from {@link #toEnumVarName}. We concatenate with {@code ::} so templates emit valid PHP (e.g.
+     * {@code \Vendor\Model\Status::AVAILABLE}).
+     *
+     * @param value    enum case name (e.g. {@code AVAILABLE})
+     * @param datatype enum class as in generated PHP (often leading {@code \} + FQCN)
+     * @return PHP default expression for that case
+     */
     @Override
     public String toEnumDefaultValue(String value, String datatype) {
-        return "self::" + datatype + "_" + value;
+        return datatype + "::" + value;
     }
 
     @Override

modules/openapi-generator/src/test/java/org/openapitools/codegen/php/PhpSymfonyServerCodegenTest.java

@@ -17,8 +17,11 @@
 
 package org.openapitools.codegen.php;
 
+import io.swagger.v3.oas.models.media.Schema;
 import org.openapitools.codegen.ClientOptInput;
 import org.openapitools.codegen.CodegenConstants;
+import org.openapitools.codegen.CodegenModel;
+import org.openapitools.codegen.CodegenProperty;
 import org.openapitools.codegen.DefaultGenerator;
 import org.openapitools.codegen.TestUtils;
 import org.openapitools.codegen.config.CodegenConfigurator;
@@ -328,6 +331,44 @@ public void testOptionalEnumRefQueryParameterWithDefaultAppliesOpenApiSemantics(
         output.deleteOnExit();
     }
 
+    /**
+     * Model property: string enum {@code $ref} with sibling {@code default} must produce a valid PHP 8.1 backed-enum
+     * default expression ({@code Type::CASE}), not {@code self::} + FQCN + {@code _CASE} from
+     * {@link AbstractPhpCodegen#toEnumDefaultValue}.
+     * <p>
+     * Spec: {@code src/test/resources/3_1/php-symfony/optional-enum-query-ref-default.yaml} ({@code Pet.HTTP.CreatePetRequest.status}).
+     */
+    @Test
+    public void testModelPropertyEnumRefWithDefaultUsesNativeEnumCaseInCodegen() {
+        final var openAPI = TestUtils.parseFlattenSpec(
+                "src/test/resources/3_1/php-symfony/optional-enum-query-ref-default.yaml");
+        final PhpSymfonyServerCodegen codegen = new PhpSymfonyServerCodegen();
+        codegen.setOpenAPI(openAPI);
+        codegen.processOpts();
+
+        Schema createPet = openAPI.getComponents().getSchemas().get("Pet.HTTP.CreatePetRequest");
+        Assert.assertNotNull(createPet, "Fixture must define Pet.HTTP.CreatePetRequest");
+
+        CodegenModel cm = codegen.fromModel("Pet.HTTP.CreatePetRequest", createPet);
+        codegen.postProcessModels(TestUtils.createCodegenModelWrapper(cm));
+
+        CodegenProperty status = cm.getVars().stream()
+                .filter(v -> "status".equals(v.getName()))
+                .findFirst()
+                .orElseThrow(() -> new AssertionError("Expected status property on CreatePetRequest model"));
+
+        Assert.assertNotNull(
+                status.getDefaultValue(),
+                "Enum ref + OpenAPI default should set CodegenProperty.defaultValue (see AbstractPhpCodegen.toDefaultValue"
+                        + " ref+default handling and updateCodegenPropertyEnum)");
+        Assert.assertFalse(
+                status.getDefaultValue().startsWith("self::"),
+                "Invalid PHP: backed enum default must not use self:: prefix, got: " + status.getDefaultValue());
+        Assert.assertTrue(
+                status.getDefaultValue().contains("::AVAILABLE"),
+                "Expected PetModelPetStatus::AVAILABLE (or FQCN::AVAILABLE), got: " + status.getDefaultValue());
+    }
+
     /**
      * Runs {@code php -l} on the file. Skips if {@code php} is not available (optional toolchain).
      */

modules/openapi-generator/src/test/resources/3_1/php-symfony/optional-enum-query-ref-default.yaml

@@ -1,13 +1,32 @@
-# Minimal OpenAPI 3.1 spec: optional query with enum $ref + default (components.parameters).
-# Repro pattern: omitting the query key should behave like the declared default; php-symfony
-# may still validate null as enum type before business logic. See project troubleshooting doc.
+# Minimal OpenAPI 3.1 spec (pet store style):
+# - Optional query: enum $ref + default (components.parameters) — regression for query/controller path.
+# - Model property: enum $ref + sibling default on an object schema — invalid PHP may be emitted for the
+#   property initializer (e.g. self::..._CASE) when default flows through AbstractPhpCodegen.toEnumDefaultValue.
 openapi: 3.1.0
 info:
   title: Optional enum query ref with default (php-symfony repro)
   version: '1.0'
+servers:
+  - url: https://petstore.example.com/api
+# Explicit empty security for minimal test fixtures (no global auth).
+security: []
 paths:
+  /pets:
+    post:
+      summary: Create a pet (request body uses enum ref + default on a property)
+      operationId: createPet
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/Pet.HTTP.CreatePetRequest'
+      responses:
+        '201':
+          description: Created
   /pets/feed-hints:
     get:
+      summary: List feed hints (optional enum query ref + default)
       operationId: listFeedHints
       parameters:
         - $ref: '#/components/parameters/ToneQuery'
@@ -35,6 +54,19 @@ components:
         $ref: '#/components/schemas/PetAnnouncementTone'
         default: friendly
   schemas:
+    Pet.Model.PetStatus:
+      type: string
+      enum:
+        - available
+        - pending
+        - sold
+    Pet.HTTP.CreatePetRequest:
+      type: object
+      properties:
+        # OAS 3.1: sibling keys beside $ref are allowed.
+        status:
+          $ref: '#/components/schemas/Pet.Model.PetStatus'
+          default: available
     PetAnnouncementTone:
       type: string
       enum: