Variables are assigned NULL when used as parameters in function by reference

Good.

I want to assign attributes to a class from an array that may or may not have the keys, like in this example:

class ClasePrueba extends Model {
    public function fillWithData(array $data){
        $this->ATRIBUTE1 = $data['data1'];
        $this->ATRIBUTE2 = $data['data2'];
        $this->ATRIBUTE3 = $data['data3'];
        $this->ATRIBUTE4 = $data['data4'];
    }
}

but if the key data1or any of the others don't exist, then I get an error of access to index not defined.

Notice: Undefined offset: data1

You could try a code like the following

$this->ATRIBUTE1 = (isset($data['data1'])? $data['data1']: NULL;

But I have noticed that the attribute is created and left as NULL. So I try an "assign" method that passes by reference the model attribute and the reference to the array element that may or may not be there, this is an example:

<?php

$array = array("1" => "PHP");

function assign(&$to, &$data){
    if(isset($data)){
        $to = $data;
    }
}

class a {
}

$testClass = new a;
var_dump(isset($testClass->attribute )); // bool(false)
var_dump($testClass->attribute); // Notice:  Undefined property: a::attribute

assign($testClass->attribute, $array["5"]);

var_dump($testClass->attribute); // NULL
var_dump(is_null($testClass->attribute )); // bool(true)
var_dump(isset($testClass->attribute )); // bool(false)
var_dump($testClass);

So when I call the first one var_dumpthe result is expected, the variable $testClass->attributeis not defined.

Then on the second I try to access, but the PHP interpreter notifies me that the variable $testClass->attributeis not defined, also as expected.

I call the function assingwith the class attribute as a parameter and with $array["5"]it being an undefined key, therefore the assignment inside the function is never executed assign, and therefore the object property is not created (in theory)

Then everything explodes for me when I call the third time var_dumpand it returns NULL. I don't understand why it returns NULL in the following calls instead of notifying that the attribute in the instance $testClassis not defined. It is as if the attribute were assigned NULL in the following way:

$testClass = new a;
var_dump(isset($testClass->attribute )); // bool(false)

$testClass->attribute = NULL;
var_dump($testClass->attribute); // NULL

and the last var_dump() returns:

object(a)#1 (1) {
    ["attribute"]=>
    NULL<br>
}

So it is as if my variables are set to NULL when using them as parameters in a function by reference, instead of reporting that they are not defined as if I were using unset(), since I 'technically' never set them to NULL.

This is my question: why is this happening?