<?php
class Counter
{
    protected $_i = 1;

    public function __toString()
    {
        return (string) $this->_i++;
    }
}

$counter = new Counter;
echo $counter . PHP_EOL; // 1
echo $counter . PHP_EOL; // 2
echo $counter . PHP_EOL; // 3
echo $counter . PHP_EOL; // 4
echo $counter . PHP_EOL; // 5