php - How can I access a property with an invalid name?

I'm adding data to a stdClass object that is going to be sent through a 3rd party API and so the names I am giving to the elements of this object are actually being defined by that external service.

$insertArray = array();
$insertArray[0] = new stdclass();
$insertArray[0]->Name = $name;

$insertArray[0]->PhoneNumber = $phone;

This was all working wonderfully until I came across a property with an invalid name:

$insertArray[0]->First.Name = $firstname;

So that isn't valid PHP syntax, so is there a way around this?