Hi,

I have class "database" which connect to db. This class have method "sql" which return the result of sql query.

PHP Code:

class Database
{
        //connect to server, select db
        function __construct() {
        ...
        }

        //run sql
        function sql($sql_query) {
        ...
        }
} 


Now I want to use object of database as variable in other class and get error.

PHP Code:

class Webpage
{
        //connect to server, select db
        function __construct() {
         ...
        $this->db = new database($params);
         }
          function write_log() {
         ...
        $this->db->sql("insert ....");
         }
} 


How can I use object database as parameter of other class?

Did you include your database class? If not you may want to look into autoloading:
http://php.net/manual/en/language.oop5.autoload.php

And, if Matt's suggestion doesn't get you going, then tell us the error.

Just a sidenote, instead of your current pattern of development, you may want to consider making the $db parameter a class field of Database, then using Webpage to extend Database, calling the parent constructor within the class constructor so that the database is already initialized within Webpage with little additional code needed.