Roman To Int in PHP

RomanToInt.php

<?php
class Solution {
    private $romans = [
        'I' => 1,
        'V' => 5,
        'X' => 10,
        'L' => 50,
        'C' => 100,
        'D' => 500,
        'M' => 1000
    ];

    /**
     * @param String $s
     * @return Integer
     */
    function romanToInt($s) {
        $sum = 0;
        $s_len = strlen($s);
        
        for($i=0 ; $i<$s_len ; $i++){
            if($this->romans[$s[$i]] < $this->romans[$s[$i+1]]){
                $sum += $this->romans[$s[$i+1]] - $this->romans[$s[$i]];
                $i++;
                continue;
            }
            $sum += $this->romans[$s[$i]];
        }
        return $sum;
    }
}