I have the same problem but I solve this error by debugging the code.
Go to the inside phpwebdriver folder and find the file webElement.php. In-line number 28 you will get the code:-
parent::__construct($root . "/element/" . $element->ELEMENT);
Replace this line to:-
foreach ($element as $ele){
parent::__construct($root . "/element/" . $ele);
}
And run the run your test file again. The error will get solved.