how does php handle objects in memory

I was trying to understand how does PHP handle objects under the hood. so i ran this small test script This basically

1. creates a new object,
2. save it to an array,
3. return the array element -containing the object- to be saved in other variable.
4. manipulate object and see if both variable and array objects will be same.

class a
{
    public $a=0;
}
class factory
{
    public $repository=[];

    function make($name){
        $this->repository[]= new $name;
        return end($this->repository);
    }
}

$f = new factory;
$a = $f->make('a'); //a Object([a]=>0);

$a->a = 6;
print_r($f->repository);  //a Object([a] => 6)
print_r($a);        //a Object([a] => 6)

$f->repository[0]->a = 9;

print_r($f->repository);  //a Object([a] => 9)
print_r($a);        //a Object([a] => 9) !

$f->repository[0] = null;
print_r($a);        //a Object([a] => 9) still. SO WHERE DOES THE OBJECT LIVES ?

The Result was that both $a and $this->repository[0] kept sync, any change in state of $a is reflected on repository array element and vise versa.

Yet if i unset $this->repository[0] $a is unaffected (although repo array was the source that created the object in first place).

I feal like i'm missing the basics here, so can some one elaborate how does php handle objects in memory, what happens when you pass object to a variable or a function ?

Note:

i'm aware of object Clone/destruction, my question is not a How to clone or copy question, its about How Does object move around in code, where does it lives, and what am i actually assigning when i assign an object to a variable ?

thanks :)