统计字符串中相同字母出现的次数并以字符串输出

function get_num($str){
    $str_arr = str_split($str);// 字符串转数组
    $x = '';// 字符串转为数组，数组中n-1位置的值
    $string = '';// 结果字符串
    foreach ($str_arr as $k => $v) {
      if($x == $v){
        $sub_str = substr($string, -2);
        $sub_str_arr = str_split($sub_str);
        $z = intval($sub_str_arr[1]) + 1;
        $string = substr($string, 0, -2).$sub_str_arr[0].$z;
      }else{
        $z = 1;
        $string = $string.$v.$z;
      }
      $x = $v;
    }
    return $string;
}

$str = 'abbaacccddddbbbf';
$res = get_num($str);
echo "<pre>";
var_dump($res);// a1b2a2c3d4b3f1