The documentation for invoke_first and invoke_last says, in that order:

mixed Functional\invoke_first(array|Traversable $collection, string $methodName[, array $methodArguments]) Invokes method $methodName on the first object in the $collection and returns the results of the call

and

mixed Functional\invoke_last(array|Traversable $collection, string $methodName[, array $methodArguments]) Invokes method $methodName on the last object in the $collection and returns the results of the call

However, the implementation of these functions seems to say otherwise. Take invoke_first as an example:

function invoke_first($collection, $methodName, array $arguments = [])
{
    //... uninteresting assertions

    foreach ($collection as $element) {

        $callback = [$element, $methodName];
        if (is_callable($callback)) {
            return $callback(...$arguments);
        }
    }

    return null;
}

This means we're calling $methodName on the first applicable object in the collection. Which means, for example, we have ['foo', 'bar', $objectWithMethodName], then $objectWithMethodName->$methodName() will be called instead of 'foo'->$methodName(), and the result of $objectWithMethodName->$methodName() will be returned instead of NULL.