<?php

/*
 * Before PHP 5.6.0
 */
function sum()
{
    $result = 0;

    foreach (func_get_args() as $i) {
        $result += $i; // assumption that $i is always an integer
    }

    return $result;
}

// Examplary call of sum()
echo sum(); // returns 0
echo sum(1, 2); // returns 3
echo sum(4, 4, 5, 7); // returns 20

/*
 * PHP 5.6.0 or later
 */
function sum(...$values) {
    $result = 0;

    foreach ($values as $i) {
        $result += $i; // assumption that $i is always an integer
    }

    return $result;
}

/*
 * PHP 5.6.0 or later including type hinting
 */
class IntObject
{
    private $value;

    public function __construct($value) {
        $this->value = $value;
    }

    public function getValue() {
        return $this->value;
    }

}

function sum(IntObject ...$values) {
    $result = 0;

    /** @var $i IntObject */
    foreach ($values as $i) {
        // getValue() can be called without any issues, since PHP already checked that all values in $values are of the type IntObject
        $result += $i->getValue();
    }

    return $result;
}

// Examplary call of sum()
var_dump(sum(new IntObject(1), new IntObject(2), new IntObject(3)));