Roman Numbers

$vals = array('I' => 1, 'V' => 5, 'X' => 10, 'L' => 50, 'C' => 100, 'D' => 500, 'M' => 1000);

function solution ($roman) {
  global $vals;
  $number = 0;
  $prev = 0;
  $chars = array_reverse(str_split($roman));
  foreach ($chars as $a)
  {
    if ($vals[$prev] > $vals[$a])
      $number -= $vals[$a];
    else
      $number += $vals[$a];

      $prev = $a;
  }
  return $number;
}