isset()/empty() not working correctly with chained references

[viktorasp]

If you are are poor soul stuck with an old code migration, please note that when migrating from PHP5 to PHP7+, the checks isset()/empty() on chained references might stop working correctly:

$user = User::Load($someId);
if (isset($user->Group->name)) {
    echo "name is set"; // PHP5
} else {
    echo "name is missing"; // PHP7+
}

After spending way too many hours on this issue, the problem is a combination of a tiny change in PHP and Doctrine1 dynamic properties usage.

PHP changes explanation:

Sandbox URL: https://onlinephp.io/c/89306

<?php
class Registry
{
    protected $_items = array();
    public function __set($key, $value)
    {
    	echo "Set: $key\n";
        $this->_items[$key] = $value;
    }
    public function __get($key)
    {
    	echo "Get: $key\n";
    	return isset($this->_items[$key]) ? $this->_items[$key] : null;
    }
    public function __isset($key)
    {
    	echo "Isset: $key\n";
        return isset($this->_items[$key]) ? empty($this->_items[$key])) === false : null;
    }
}
class RegB extends Registry {}

$registry = new Registry();
$registry->one = new RegB();
$registry->one->two = 'value';


var_dump(empty($registry->one->two)); 

Running the code above, there's a small difference: when doing check empty($registry->test->secret), PHP7+ does isset() check for each element in a chain, starting from the left side. So, it goes like this
PHP7+:
Isset: one
Get: one
Isset: two
Get: two

PHP5:
Get: one
Isset: two
Get: two

Now we are getting to the Doctrine1 part!

Doctrine/Record.php:1782 has

if (isset($this->_references[$offset]) &&
    $this->_references[$offset] !== self::$_null) {
    return true;
}

which checks if a given offset exists among references, and in PHP5 that works just fine, as we are checking the right-most value in the chain for existence. In PHP7+ this check fails, as we are checking the left-most chain member, and if it's not loaded yet, then this check will fail.

As for the possible fixes, either user-land code could change isset()/empty() to a bunch of validity and instanceof checks, in case of the original example, something like:

$user = User::Load($someId);
if ($user->Group instanceof Group && isset($user->Group->name)) {
// if ($user->Group && $user->Group->name) {
    echo "name is set"; // hooray!
} else {
    echo "name is missing";
}

optionally, Doctrine/Record::contains() could be modified to attempt loading of a reference, if it was not found immediately.