fix PHP: Fixed typing of offsetGet/offsetSet (#21583)

Author: florentausha

modules/openapi-generator/src/main/resources/php/model_generic.mustache

@@ -476,24 +476,24 @@ class {{classname}} {{#parentSchema}}extends {{{parent}}}{{/parentSchema}}{{^par
     /**
      * Returns true if offset exists. False otherwise.
      *
-     * @param integer $offset Offset
+     * @param integer|string $offset Offset
      *
      * @return boolean
      */
-    public function offsetExists($offset): bool
+    public function offsetExists(mixed $offset): bool
     {
         return isset($this->container[$offset]);
     }
 
     /**
      * Gets offset.
      *
-     * @param integer $offset Offset
+     * @param integer|string $offset Offset
      *
      * @return mixed|null
      */
     #[\ReturnTypeWillChange]
-    public function offsetGet($offset)
+    public function offsetGet(mixed $offset)
     {
         return $this->container[$offset] ?? null;
     }
@@ -518,11 +518,11 @@ class {{classname}} {{#parentSchema}}extends {{{parent}}}{{/parentSchema}}{{^par
     /**
      * Unsets offset.
      *
-     * @param integer $offset Offset
+     * @param integer|string $offset Offset
      *
      * @return void
      */
-    public function offsetUnset($offset): void
+    public function offsetUnset(mixed $offset): void
     {
         unset($this->container[$offset]);
     }