diff --git a/modules/openapi-generator/src/main/resources/php-nextgen/model_generic.mustache b/modules/openapi-generator/src/main/resources/php-nextgen/model_generic.mustache
index 06ff303fe6e..4904e3de9f6 100644
--- a/modules/openapi-generator/src/main/resources/php-nextgen/model_generic.mustache
+++ b/modules/openapi-generator/src/main/resources/php-nextgen/model_generic.mustache
@@ -419,6 +419,10 @@ class {{classname}} {{#parentSchema}}extends {{{parent}}}{{/parentSchema}}{{^par
         $allowedValues = $this->{{getter}}AllowableValues();
         {{^isContainer}}
         if ({{#isNullable}}!is_null(${{name}}) && {{/isNullable}}!in_array(${{{name}}}, $allowedValues, true)) {
+        {{#enumUnknownDefaultCase}}
+            ${{name}} = {{#allowableValues}}{{#enumVars}}{{#-last}}self::{{enumName}}_{{{name}}};{{/-last}}{{/enumVars}}{{/allowableValues}}
+        {{/enumUnknownDefaultCase}}
+        {{^enumUnknownDefaultCase}}
             throw new InvalidArgumentException(
                 sprintf(
                     "Invalid value '%s' for '{{name}}', must be one of '%s'",
@@ -426,6 +430,7 @@ class {{classname}} {{#parentSchema}}extends {{{parent}}}{{/parentSchema}}{{^par
                     implode("', '", $allowedValues)
                 )
             );
+        {{/enumUnknownDefaultCase}}
         }
         {{/isContainer}}
         {{#isContainer}}
diff --git a/modules/openapi-generator/src/main/resources/php/model_generic.mustache b/modules/openapi-generator/src/main/resources/php/model_generic.mustache
index 0dc2675f4b5..80ad599c172 100644
--- a/modules/openapi-generator/src/main/resources/php/model_generic.mustache
+++ b/modules/openapi-generator/src/main/resources/php/model_generic.mustache
@@ -408,6 +408,10 @@ class {{classname}} {{#parentSchema}}extends {{{parent}}}{{/parentSchema}}{{^par
         $allowedValues = $this->{{getter}}AllowableValues();
         {{^isContainer}}
         if ({{#isNullable}}!is_null(${{name}}) && {{/isNullable}}!in_array(${{{name}}}, $allowedValues, true)) {
+        {{#enumUnknownDefaultCase}}
+            ${{name}} = {{#allowableValues}}{{#enumVars}}{{#-last}}self::{{enumName}}_{{{name}}};{{/-last}}{{/enumVars}}{{/allowableValues}}
+        {{/enumUnknownDefaultCase}}
+        {{^enumUnknownDefaultCase}}
             throw new \InvalidArgumentException(
                 sprintf(
                     "Invalid value '%s' for '{{name}}', must be one of '%s'",
@@ -415,6 +419,7 @@ class {{classname}} {{#parentSchema}}extends {{{parent}}}{{/parentSchema}}{{^par
                     implode("', '", $allowedValues)
                 )
             );
+        {{/enumUnknownDefaultCase}}
         }
         {{/isContainer}}
         {{#isContainer}}
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/php/PhpClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/php/PhpClientCodegenTest.java
index d462e61fd94..66a6302344b 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/php/PhpClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/php/PhpClientCodegenTest.java
@@ -17,16 +17,33 @@
 
 package org.openapitools.codegen.php;
 
+import io.swagger.parser.OpenAPIParser;
 import io.swagger.v3.oas.models.OpenAPI;
-import org.openapitools.codegen.CodegenConstants;
-import org.openapitools.codegen.CodegenModel;
+import io.swagger.v3.parser.core.models.ParseOptions;
+import org.openapitools.codegen.*;
+import org.openapitools.codegen.java.assertions.JavaFileAssert;
+import org.openapitools.codegen.languages.JavaMicroprofileServerCodegen;
 import org.openapitools.codegen.languages.PhpClientCodegen;
-import org.openapitools.codegen.TestUtils;
 import org.testng.Assert;
+import org.testng.annotations.BeforeMethod;
 import org.testng.annotations.Test;
 
+import java.io.File;
+import java.nio.file.Files;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
 public class PhpClientCodegenTest {
 
+    protected PhpClientCodegen codegen;
+
+    @BeforeMethod
+    public void before() {
+        codegen = new PhpClientCodegen();
+    }
+
     @Test
     public void testInitialConfigValues() throws Exception {
         final PhpClientCodegen codegen = new PhpClientCodegen();
@@ -90,4 +107,61 @@ public class PhpClientCodegenTest {
         Assert.assertEquals(simpleName.classname, "DollarModel");
         Assert.assertEquals(simpleName.classVarName, "dollar_model");
     }
+
+    @Test
+    public void testEnumUnknownDefaultCaseDeserializationEnabled() throws Exception {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+
+        OpenAPI openAPI = new OpenAPIParser()
+                .readLocation("src/test/resources/bugs/issue_20593.yaml", null, new ParseOptions()).getOpenAPI();
+
+        codegen.setOutputDir(output.getAbsolutePath());
+        codegen.additionalProperties().put(CodegenConstants.ENUM_UNKNOWN_DEFAULT_CASE, "true");
+
+        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("Pet.php").toPath())
+                .stream()
+                .map(String::trim)
+                .collect(Collectors.toList());
+
+        Assert.assertListContains(modelContent, a -> a.equals("$color = self::COLOR_UNKNOWN_DEFAULT_OPEN_API;"), "");
+        Assert.assertListNotContains(modelContent, a -> a.equals("\"Invalid value '%s' for 'color', must be one of '%s'\","), "");
+    }
+
+    @Test
+    public void testEnumUnknownDefaultCaseDeserializationDisabled() throws Exception {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+
+        OpenAPI openAPI = new OpenAPIParser()
+                .readLocation("src/test/resources/bugs/issue_20593.yaml", null, new ParseOptions()).getOpenAPI();
+
+        codegen.setOutputDir(output.getAbsolutePath());
+
+        ClientOptInput input = new ClientOptInput()
+                .openAPI(openAPI)
+                .config(codegen);
+
+        DefaultGenerator generator = new DefaultGenerator();
+        Map<String, File> files = generator.opts(input).generate().stream()
+                .collect(Collectors.toMap(File::getName, Function.identity()));
+
+        List<String> modelContent = Files
+                .readAllLines(files.get("Pet.php").toPath())
+                .stream()
+                .map(String::trim)
+                .collect(Collectors.toList());
+
+        Assert.assertListNotContains(modelContent, a -> a.equals("$color = self::COLOR_UNKNOWN_DEFAULT_OPEN_API;"), "");
+        Assert.assertListContains(modelContent, a -> a.equalsIgnoreCase("\"Invalid value '%s' for 'color', must be one of '%s'\","), "");
+    }
 }
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/php/PhpNextgenClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/php/PhpNextgenClientCodegenTest.java
index e8c7c3c88f6..16ed6aeb0d7 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/php/PhpNextgenClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/php/PhpNextgenClientCodegenTest.java
@@ -16,13 +16,35 @@
 
 package org.openapitools.codegen.php;
 
+import io.swagger.parser.OpenAPIParser;
+import io.swagger.v3.oas.models.OpenAPI;
+import io.swagger.v3.parser.core.models.ParseOptions;
+import org.openapitools.codegen.ClientOptInput;
+import org.openapitools.codegen.CodegenConstants;
+import org.openapitools.codegen.DefaultGenerator;
+import org.openapitools.codegen.languages.PhpClientCodegen;
 import org.openapitools.codegen.languages.PhpNextgenClientCodegen;
 import org.openapitools.codegen.testutils.ConfigAssert;
 import org.testng.Assert;
+import org.testng.annotations.BeforeMethod;
 import org.testng.annotations.Test;
 
+import java.io.File;
+import java.nio.file.Files;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
 public class PhpNextgenClientCodegenTest {
 
+    protected PhpNextgenClientCodegen codegen;
+
+    @BeforeMethod
+    public void before() {
+        codegen = new PhpNextgenClientCodegen();
+    }
+
     @Test
     public void testInitialConfigValues() throws Exception {
         final PhpNextgenClientCodegen codegen = new PhpNextgenClientCodegen();
@@ -68,4 +90,61 @@ public class PhpNextgenClientCodegenTest {
         configAssert.assertValue(PhpNextgenClientCodegen.SUPPORT_STREAMING, codegen::isSupportStreaming, Boolean.TRUE);
         Assert.assertEquals(codegen.isSupportStreaming(), true);
     }
+
+    @Test
+    public void testEnumUnknownDefaultCaseDeserializationEnabled() throws Exception {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+
+        OpenAPI openAPI = new OpenAPIParser()
+                .readLocation("src/test/resources/bugs/issue_20593.yaml", null, new ParseOptions()).getOpenAPI();
+
+        codegen.setOutputDir(output.getAbsolutePath());
+        codegen.additionalProperties().put(CodegenConstants.ENUM_UNKNOWN_DEFAULT_CASE, "true");
+
+        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("Pet.php").toPath())
+                .stream()
+                .map(String::trim)
+                .collect(Collectors.toList());
+
+        Assert.assertListContains(modelContent, a -> a.equals("$color = self::COLOR_UNKNOWN_DEFAULT_OPEN_API;"), "");
+        Assert.assertListNotContains(modelContent, a -> a.equals("\"Invalid value '%s' for 'color', must be one of '%s'\","), "");
+    }
+
+    @Test
+    public void testEnumUnknownDefaultCaseDeserializationDisabled() throws Exception {
+        File output = Files.createTempDirectory("test").toFile().getCanonicalFile();
+        output.deleteOnExit();
+
+        OpenAPI openAPI = new OpenAPIParser()
+                .readLocation("src/test/resources/bugs/issue_20593.yaml", null, new ParseOptions()).getOpenAPI();
+
+        codegen.setOutputDir(output.getAbsolutePath());
+
+        ClientOptInput input = new ClientOptInput()
+                .openAPI(openAPI)
+                .config(codegen);
+
+        DefaultGenerator generator = new DefaultGenerator();
+        Map<String, File> files = generator.opts(input).generate().stream()
+                .collect(Collectors.toMap(File::getName, Function.identity()));
+
+        List<String> modelContent = Files
+                .readAllLines(files.get("Pet.php").toPath())
+                .stream()
+                .map(String::trim)
+                .collect(Collectors.toList());
+
+        Assert.assertListNotContains(modelContent, a -> a.equals("$color = self::COLOR_UNKNOWN_DEFAULT_OPEN_API;"), "");
+        Assert.assertListContains(modelContent, a -> a.equalsIgnoreCase("\"Invalid value '%s' for 'color', must be one of '%s'\","), "");
+    }
 }
diff --git a/modules/openapi-generator/src/test/resources/bugs/issue_20593.yaml b/modules/openapi-generator/src/test/resources/bugs/issue_20593.yaml
new file mode 100644
index 00000000000..2e8a81c1129
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/bugs/issue_20593.yaml
@@ -0,0 +1,20 @@
+openapi: "3.0.0"
+info:
+  version: 2.0.0
+  title: test
+paths:
+  /pets:
+    get:
+      summary: List all pets
+      operationId: listPets
+      responses:
+        '200':
+          description: OK
+components:
+  schemas:
+    Pet:
+      type: object
+      properties:
+        Color:
+          type: string
+          enum: [RED, BLUE, GREEN]
\ No newline at end of file