diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
index 1d530d859fd..d75c03b34e7 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
@@ -3941,7 +3941,10 @@ public class DefaultCodegen implements CodegenConfig {
     }
 
     protected String getCollectionFormat(Parameter parameter) {
-        if (Parameter.StyleEnum.FORM.equals(parameter.getStyle())) {
+        if (
+               Parameter.StyleEnum.FORM.equals(parameter.getStyle())
+            || Parameter.StyleEnum.SIMPLE.equals(parameter.getStyle())
+           ) {
             if (parameter.getExplode() != null && parameter.getExplode()) {
                 return "csv";
             } else {
diff --git a/modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml b/modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml
index 073e87be9e9..7da6336ee20 100644
--- a/modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml
+++ b/modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml
@@ -580,6 +580,7 @@ paths:
       parameters:
         - name: enum_header_string_array
           in: header
+          explode: true
           description: Header parameter enum test (string array)
           schema:
             type: array
diff --git a/samples/openapi3/client/petstore/php/SwaggerClient-php/lib/Api/FakeApi.php b/samples/openapi3/client/petstore/php/SwaggerClient-php/lib/Api/FakeApi.php
index 289266c9610..7fad7d64764 100644
--- a/samples/openapi3/client/petstore/php/SwaggerClient-php/lib/Api/FakeApi.php
+++ b/samples/openapi3/client/petstore/php/SwaggerClient-php/lib/Api/FakeApi.php
@@ -1933,6 +1933,9 @@ class FakeApi
             $queryParams['enum_query_double'] = ObjectSerializer::toQueryValue($enum_query_double);
         }
         // header params
+        if (is_array($enum_header_string_array)) {
+            $enum_header_string_array = ObjectSerializer::serializeCollection($enum_header_string_array, 'csv');
+        }
         if ($enum_header_string_array !== null) {
             $headerParams['enum_header_string_array'] = ObjectSerializer::toHeaderValue($enum_header_string_array);
         }