$cable_a = 'R8,U5,L5,D3';
$cable_b = 'U7,R6,D4,L4';

$min_distance = -1;

function travelCable($cable, &$raster, &$intersections, $indicator)
{
  $pointer_x = 0;
  $pointer_y = 0;

  $raster[$pointer_x][$pointer_y] = 'o';

  foreach (explode(',', $cable) as $instruction) {
    $direction = substr($instruction, 0, 1);
    $amount = substr($instruction, 1);

    for ($a = 0; $a < $amount; $a++) {
      switch ($direction) {
        case 'U':
          $pointer_y++;
          break;
        case 'D':
          $pointer_y--;
          break;
        case 'L':
          $pointer_x--;
          break;
        case 'R':
          $pointer_x++;
          break;
      }
      if (!empty($raster[$pointer_x][$pointer_y])) {
        $intersections[$pointer_x][$pointer_y] = 1;
      } else {
        $raster[$pointer_x][$pointer_y] = $indicator;
      }
    }
  }
}

travelCable($cable_a, $raster, $intersections, 'a');
travelCable($cable_b, $raster, $intersections, 'b');

function distance($vector1, $vector2)
{
  $n = count($vector1);
  $sum = 0;
  for ($i = 0; $i < $n; $i++) {
    $sum += abs($vector1[$i] - $vector2[$i]);
  }
  return $sum;
}

foreach ($intersections as $x => $intersection) {
  foreach ($intersection as $y => $value) {
    $distance = distance([0, 0], [$x, $y]);
    if ($min_distance < 0 || $distance < $min_distance) {
      $min_distance = $distance;
    }
    $intersections[$x][$y] = $distance;
    $raster[$x][$y] = 'x';
  }
}