fix(php): Union references render as class names and not mixed

generators/php/base/src/context/PhpTypeMapper.ts

@@ -132,9 +132,8 @@ export class PhpTypeMapper {
             case "enum":
                 return preserveEnums ? php.Type.reference(classReference) : php.Type.enumString(classReference);
             case "object":
-                return php.Type.reference(classReference);
             case "union":
-                return php.Type.mixed();
+                return php.Type.reference(classReference);
             case "undiscriminatedUnion": {
                 return php.Type.union(
                     // need to dedupe because lists and sets are both represented as array

generators/php/sdk/versions.yml

@@ -1,3 +1,10 @@
+- version: 0.13.1
+  changelogEntry:
+    - type: fix
+      summary: >-
+        Render union references as their type references rather than mixed.
+  irVersion: 55
+
 - version: 0.13.0
   changelogEntry:
     - type: feat