[php][php-nextgen] Fix nullability when multiple response types are possible (#22827)

* [php][php-nextgen] Fix nullability when multiple response types are possible

* [php][php-nextgen] Fix test

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

@@ -214,7 +214,11 @@ public class PhpNextgenClientCodegen extends AbstractPhpCodegen {
                 String phpReturnType = String.join("|", phpReturnTypeOptions);
                 String docReturnType = String.join("|", docReturnTypeOptions);
                 if (hasEmptyResponse) {
-                    phpReturnType = "?" + phpReturnType;
+                    if (phpReturnTypeOptions.size() > 1) {
+                        phpReturnType = phpReturnType + "|null";
+                    } else {
+                        phpReturnType = "?" + phpReturnType;
+                    }
                     docReturnType = docReturnType + "|null";
                 }
 

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

@@ -29,6 +29,7 @@ import org.testng.annotations.BeforeMethod;
 import org.testng.annotations.Test;
 
 import java.io.File;
+import java.io.IOException;
 import java.nio.file.Files;
 import java.util.List;
 import java.util.Map;
@@ -146,4 +147,33 @@ public class PhpNextgenClientCodegenTest {
         Assert.assertListNotContains(modelContent, a -> a.equals("$color = self::COLOR_UNKNOWN_DEFAULT_OPEN_API;"), "");
         Assert.assertListContains(modelContent, a -> a.equalsIgnoreCase("\"Invalid value '%s' for 'color', must be one of '%s'\","), "");
     }
+
+    @Test
+    public void testDifferentResponseSchemasWithEmpty() throws IOException {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+
+        OpenAPI openAPI = new OpenAPIParser()
+                .readLocation("src/test/resources/bugs/issue_22817.yaml", null, new ParseOptions())
+                .getOpenAPI();
+
+
+        codegen.setOutputDir(output.getAbsolutePath());
+        ClientOptInput input = new ClientOptInput()
+                .openAPI(openAPI)
+                .config(codegen);
+
+        DefaultGenerator generator = new DefaultGenerator();
+        Map<String, File> files = generator.opts(input).generate().stream()
+                .collect(Collectors.toMap(File::getName, Function.identity()));
+
+        List<String> modelContent = Files
+                .readAllLines(files.get("DefaultApi.php").toPath())
+                .stream()
+                .map(String::trim)
+                .collect(Collectors.toList());
+
+        Assert.assertListContains(modelContent, a -> a.equals("): int|string|null"), "Expected to find nullable return type declaration.");
+        Assert.assertListNotContains(modelContent, a -> a.equals("): ?int|string"), "Expected to not find invalid union type with '?'.");
+    }
 }

modules/openapi-generator/src/test/resources/bugs/issue_22817.yaml

@@ -0,0 +1,24 @@
+openapi: 3.0.4
+info:
+  title: "Different response schemas including an empty one"
+  version: "1.0.0"
+paths:
+  /example:
+    get:
+      operationId: exampleGet
+      responses:
+        200:
+          description: "A successful response with data"
+          content:
+            application/json:
+              schema:
+                type: integer
+        400:
+          description: "A bad request with a message"
+          content:
+            application/json:
+              schema:
+                type: string
+        500:
+          description: "An internal server error with no content"
+          content: { }