https://github.com/EdeMeijer/serialize-debugger is also available.

ValidatePhpSerialization.php

<?php

  /**
   * Checks where a serialize call would've failed.
   */
  public static function check($checking_key, $data, $path = [], $processed = []) {
    try {
      // Attempt to serialize the data to check for serializability.
      serialize($data);
    } catch (\Throwable $e) {
      // Serialization failed, track the issue.
      $path[] = "CATCH $checking_key";

      // Handle arrays recursively.
      if (is_array($data)) {
        foreach ($data as $key => $val) {
          $path[$key] = static::check($key, $val, [], $processed);
        }
      } elseif (is_object($data)) {
        // Generate a unique hash for the object.
        $object_hash = spl_object_hash($data);

        // Check if we've already processed this object to prevent recursion.
        if (in_array($object_hash, $processed, true)) {
          $path[] = "RECURSION $checking_key";
          return $path; // Bail out on recursion.
        }

        // Add the object hash to the processed list.
        $processed[] = $object_hash;

        $ref = new \ReflectionObject($data);
        $properties = [];

        // Get properties, including private ones from parent classes.
        do {
          $properties = array_merge($properties, $ref->getProperties());
          $ref = $ref->getParentClass();
        } while ($ref);

        foreach ($properties as $property) {
          $property->setAccessible(true);
          $key = $property->getName();
          if (!$property->isInitialized($data)) {
            continue;
          }
          try {
            $val = $property->getValue($data);
          }
          catch (\Exception $access_error) {
            // Ignore it.
            continue;
          }
          $path[$key] = static::check($key, $val, [], $processed);
        }
      } else {
        // Final case for non-object and non-array data.
        $path[] = "FINAL $checking_key";
      }
    }

    return array_filter($path);
  }