Can I instantiate a PHP class inside another class?

I was wondering if it is allowed to create an instance of a class inside another class.

Or, do I have to create it outside and then pass it in through the constructor? But then I would have created it without knowing if I would need it.

Example (a database class):

class some{

if(.....){
include SITE_ROOT . 'applicatie/' . 'db.class.php';
$db=new db

You can't define a class in another class. You should include files with other classes outside of the class. In your case, that will give you two top-level classes db and some. Now in the constructor of some you can decide to create an instance of db. For example:

include SITE_ROOT . 'applicatie/' . 'db.class.php';

class some {

    public function __construct() {
        if (...) {
            $this->db = new db;
        }
    }

}

People saying that it is possible to 'create a class within a class' seem to mean that it is possible to create an object / instance within a class. I have not seen an example of an actual class definition within another class definition, which would look like:

class myClass{

    class myNestedClass{

    }

}

/* THIS IS NOT ALLOWED AND SO IT WON'T WORK */

Since the question was 'is it possible to create a class inside another class', I can now only assume that it is NOT possible.

I just wanted to point out that it is possible to load a class definition dynamically inside another class definition.

Lukas is right that we cannot define a class inside another class, but we can include() or require() them dynamically, since every functions and classes defined in the included file will have a global scope. If you need to load a class or function dynamically, simply include the files in one of the class' functions. You can do this:

function some()
{
    require('db.php');
    $db = new Db();
    ...
}

http://php.net/manual/en/function.include.php

It's impossible to create a class in another class in PHP. Creating an object(an instance of a class) in another object is a different thing.

No, it is not possible. Nesting classes and aggregating objects are diffrent mechanisms. Nesting classes means nesting class names, in fact. Class A nested in class B would be named "B\A" (?). From v5.3 you can nest class names (and some other names) in namespaces.

Can I instantiate a PHP class inside another class?

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