php oop call a class within a class [duplicate]

This question already has answers here: Call to a member function on a non-object [duplicate] (8 answers) Closed 10 years ago.

I need help trying to call a class function within another class. Here's my setup:

db.php

class db
{
function connect(){
//db connection code here
}
function query($sql){
//perform query and return results
}
}//end db class

main.php

class main
{
function test(){
$sql = "select * from user";
$result = $db->query($sql);//does not work
}
}//end class

index.php

require_once './db.php';
$db=new database;
$db->connect();
//so far so good
require_once './main.php';
$main=new main;
//r开发者_运维问答est of the code here

The problem is that on index.php I can run $db->query($sql) without any problems, but I can't seem to run them inside the main class, more specifically inside the test function. So if I call $main->test() from index.php I get this error:

Fatal error: Call to a member function query() on a non-object in /path/to/file

---

Unlike C global variables in PHP are not in the visibility scope of functions/methods, see http://docs.php.net/language.variables.scope
To access global variables you must either import them into the function's scope via global $var or acccess them through the superglobal $_GLOBAL

But instead of using global variables you might be interested in dependency injection, e.g. (one of many possible ways to implement di):

<?php
$db = new db;
$db->connect();
$main = new main($db);
$result = $main->test();

class db
{
    function connect(){
        //db connection code here
    }
    function query($sql){
        //perform query and return results
    }
}


class main
{
    protected $db;
    public function __construct($db) {
        $this->db = $db;
    }

    public function test(){
        $sql = "select * from user";
        return $this->db->query($sql);
    }
}

---

I would add a static getInstance() method into the Db class. Then cou can call Db::getInstance()->query(...) from every scope. You can read more on this pattern on Wikipedia.

---

Just put:

global $db;

in your test() function.

But if you do this also make sure you check that $db exists and is what you think it is before you use it this way.

---

I guess you should be using

$db = new db;

because $db is not defined in your main class.