Why does PHP think I want to convert that object to a string?

```php
class Class1 {}

class Class2 {
    function __construct( Class1 $class1Obj) {
        $this->$class1Obj = $class1Obj; // -> Catchable fatal error: Object of class Class1 could not be converted to string
    }
}

$class1Obj = new Class1();
$class2Obj = new Class2($class1Obj);

```