override.php

<?php

class x {
    public $sum = 0;

    public function __invoke($num = 0)
    {
        $this->sum += $num;
        return $this;
    }

    public function __toString()
    {
        try {
            return (string)$this->sum;
        } finally {
            $this->sum = 0;
        }
    }
}

$x = new x();

echo $x(7)(5)(3) . PHP_EOL;
echo $x(1)(2)(3)(4) . PHP_EOL;