src/includes/classes/Core/Error.php
public function data(string $slug = '', bool $by_slug = false)
    {
        if (!$slug) { // All data (i.e., for all slugs)?
            //
            if ($by_slug) { // All data keyed by slug?
                return $this->error_data ?: [$this->default_slug => null];
                //
            } else { // Data for the first slug.
                // NOTE: Different than `messages()`.
                $slug = $this->slug(); // First slug.
                return $this->error_data[$slug] ?? null;
            }
        } else {
            return $this->error_data[$slug] ?? null;
        }
    }
public function data(string $slug = '', bool $by_slug = false)
    {
        if (!$slug) { // All data (i.e., for all slugs)?
            //
            if ($by_slug) { // All data keyed by slug?
                return $this->error_data ?: [$this->default_slug => null];
                //
            } else { // Data for the first slug.
                // NOTE: Different than `messages()`.
                $slug = $this->slug(); // First slug.
                return $this->error_data[$slug] ?? null;
            }
        } else {
            return $this->error_data[$slug] ?? null;
        }
    }
Get error data.

@since 160710 Error utils.

@param string $slug    Error slug.
@param bool   $by_slug All data by slug?

@return array|mixed|null Error data.