source/php/Entity/CustomPostType.php
public function tableSortableColumns($columns)
    {
        if (!empty($this->tableSortableColumns) && is_array($this->tableSortableColumns)) {
            $columns = $this->tableColumns;
        }

        function arraytolower(array $columns, $round = 0)
        {
            return unserialize(strtolower(serialize($columns)));
        }

        return arraytolower($columns);
    }
public function tableSortableColumns($columns)
    {
        if (!empty($this->tableSortableColumns) && is_array($this->tableSortableColumns)) {
            $columns = $this->tableColumns;
        }

        function arraytolower(array $columns, $round = 0)
        {
            return unserialize(strtolower(serialize($columns)));
        }

        return arraytolower($columns);
    }
Setup sortable columns
@param  array $columns Default columns
@return array          New columns