学说记录的范围是什么？

I am investigating a bug where it looks like Doctrine does not respect the idea of scope on variables when returning Record objects.

<?php

class Thing {
    var $variable;

    public function doStuff() {
        $thing1 = self::getById(1);
        echo($thing1->variable); 
        $thing1->variable = "new value";
        Thing::doSomeOtherStuff();
        echo($thing1->variable);
    }

    public static function doSomeOtherStuff() {
        $thing2 = Thing::getById(1);
    }

    /**
     * this runs a query against a database and 
     * returns a Thing object with $variable = "old value" 
     */ 
    public static function getById($id): Thing {
        return Doctrine_Query::CREATE()
            ->select('t.*')
            ->from('Thing t')
            ->where('t.id = ?', $id) 
            ->fetchOne([], Doctrine_Core::HYDRATE_RECORD);
    }
}

expected output:

old value
new value

actual output:

old value 
old value 

The the odd behavior does not happen when using HYDRATE_ARRAY. Using a debugger it looks like this method is doing the actual updating of the value of $thing1 while creating $thing2.

I would very much like to know why this happens and why the behavior is desirable.