This is only a suggestion, but I needed time to understand examples. I suggest you to remove the object requirement. For function example :

function sum() {
$arguments = func_get_args();
$result = 0;
foreach ($arguments as $argument)
$result += $argument;
return $result;
}
$evaluator = new \Matex\Evaluator();
$evaluator->functions = [
'sum' => ['ref' => 'sum', 'arc' => null]
];

the operator ^ doesn't work yet, it is just ignored, also a documentation for how to use it would be good, the current example only shows one feature