PHP (4) "Roman numeral to Integer" algorithm problem

The original title is this: the Roman numerals contain the following seven characters: I, V, X, L,c,d and M. i=1,v=5,x=10,l=50,c=100,d=500,m=1000 For example, the Roman numeral 2 writes II, which is two parallel 1. 12 write XII, i.e. X + II. 27 Write the XXVII, that is XX + V + II. Usually, the number of Roman numerals is small and the numbers are on the right side of the large numbers. But there are special cases, such as 4 do not write IIII, but IV. The number 1 is on the left side of the number 5, the number represented is equal to the large number 5 decreases the number 1 gets the value 4. Similarly, the number 9 is represented as IX. This special rule only applies to the following six cases:

• I can be placed on the left of V (5) and X (10) to represent 4 and 9.
• X can be placed on the left of L (50) and C (100) to represent 40 and 90.
• C can be placed on the left of D (500) and M (1000) to represent 400 and 900. The
• converts it to an integer given a Roman number. The input is guaranteed to be within the range of 1 to 3999.

• Example 1:

  Input: "III"
  Output: 3

li>

Example 2:

Input: "IV"
Output: 4

• Example 1:

  Output In: "MCMXCIV"
  Output: 1994

class test
{
    public function a5($str)
        {
            $num = 0;
            $num_arr = str_split($str);
            $length = count($num_arr);
            for ($i=0; $i < $length; $i++) 
            { 
                switch ($num_arr[$i]) 
                {
                    case ‘I‘:
                            switch ($num_arr[$i+1]) 
                            {
                                case ‘V‘:
                                    $num += 4;
                                    $i++;
                                    break;
                                case ‘X‘:
                                    $num += 9;
                                    $i++;
                                    break;
                                default:
                                    $num += 1;
                                    break;
                            }
                        break;
                    case ‘V‘:
                        $num += 5;
                        break;
                    case ‘X‘:
                            switch ($num_arr[$i+1]) 
                            {
                                case ‘L‘:
                                    $num += 40;
                                    $i++;
                                    break;
                                case ‘C‘:
                                    $num += 90;
                                    $i++;
                                    break;
                                default:
                                    $num += 10;
                                    break;
                            }
                        break;
                    case ‘L‘:
                        $num += 50;
                        break;
                    case ‘C‘:
                            switch ($num_arr[$i+1]) 
                            {
                                case ‘D‘:
                                    $num += 400;
                                    $i++;
                                    break;
                                case ‘M‘:
                                    $num += 900;
                                    $i++;
                                    break;
                                default:
                                    $num += 100;
                                    break;
                            }
                        break;
                    case ‘D‘:
                        $num += 500;
                        break;
                    case ‘M‘:
                        $num += 1000;
                        break;
                    default:
                        break;
                }
            }
            echo $num;
        }
}

PHP (4) "Roman numeral to Integer" algorithm problem