PHP Facemash ELO Rating Class/Function

Question

I got the following ELO class from PHPClasses website.

<?php 
class elo_calculator {
    public function rating($S1, $S2, $R1, $R2) {
        if(empty($S1) or empty($S2) or empty($R1) or empty($R2))
            return null;
        if($S1 != $S2) {
            if($S1 > $S2) {
                $E = 120 - round(1 / (1 + pow(10, (($R2 - $R1) / 400))) * 120);
                $R['R3'] = $R1 + $E;
                $R['R4'] = $R2 - $E;
            } else {
                $E = 120 - round(1 / (1 + pow(10, (($R1 - $R2) / 400))) * 120);
                $R['R3'] = $R1 - $E;
                $R['R4'] = $R2 + $E;
            }
        } else {
            if($R1 == $R2) {
                $R['R3'] = $R1;
                $R['R4'] = $R2;
            } else {
                if($R1 > $R2) {
                    $E = (120 - round(1 / (1 + pow(10, (($R1 - $R2) / 400))) * 120)) - (120 - round(1 / (1 + pow(10, (($R2 - $R1) / 400))) * 120));
                    $R['R3'] = $R1 - $E;
                    $R['R4'] = $R2 + $E;
                } else {
                    $E = (120 - round(1 / (1 + pow(10, (($R2 - $R1) / 400))) * 120)) - (120 - round(1 / (1 + pow(10, (($R1 - $R2) / 400))) * 120));
                    $R['R3'] = $R1 + $E;
                    $R['R4'] = $R2 - $E;
                }
            }
        }
        $R['S1'] = $S1;
        $R['S2'] = $S2;
        $R['R1'] = $R1;
        $R['R2'] = $R2;
        $R['P1'] = ((($R['R3'] - $R['R1']) > 0)?"+" . ($R['R3'] - $R['R1']) : ($R['R3'] - $R['R1']));
        $R['P2'] = ((($R['R4'] - $R['R2']) > 0)?"+" . ($R['R4'] - $R['R2']) : ($R['R4'] - $R['R2']));
        return $R;
    }
} 
?>

I am trying to apply this to my food rating site.

Here is what i understand

1. To start off with the system we need to assign a base score for all the dishes.
2. We have 4 variables S1, S2, R1, R2 (S= score, R = rank)
3. When rating between two dishes if i press the first dish. what will be the the S1 and S2 ? will it be 1-0 ?
4. What if i add another dish after 10k battles ? since i will be assigning a base score for it will it fair better ?
5. How can i stop a score of a Dish not to go below 0.

Here is PHP implementation of the same. Can you help me understand the 4 variables and how should i use it ?

Answer 1

here on GitHub is the best php class for ELO rating system i've ever found: https://github.com/Chovanec/elo-rating

USAGE:

// player A elo = 1000
// player B elo = 2000
// player A lost
// player B win
$raging = new Rating(1000, 2000, 0, 1);

// player A elo = 1000
// player B elo = 2000
// player A draw
// player B draw
$raging = new Rating(1000, 2000, .5, .5);