code: public function dropForeignKey($name, $table)
      {
          return 'ALTER TABLE ' . $this->db->quoteTableName($table)
              . ' DROP CONSTRAINT ' . $this->db->quoteColumnName($name);
      }
docs: Builds a SQL statement for dropping a foreign key constraint.
  @param string $name the name of the foreign key constraint to be dropped. The name will be properly quoted by the method.
  @param string $table the table whose foreign is to be dropped. The name will be properly quoted by the method.
  @return string the SQL statement for dropping a foreign key constraint.