PHP intercepts UTF8 or GBK encoded Chinese and English strings

//String interception
$a = "s＠@Hello";
var_dump(strlen_weibo($a,'utf-8'));
Result The output is 8, in which the letter s counts as 1, the full-width @ counts as 2, the half-width @ counts as 1, and the two Chinese characters count as 4. The source code is as follows:
//Function code to intercept strings
function strlen_weibo($string, $charset='utf-8')
{
 $n = $count = 0;
 $ length = strlen($string);
 if (strtolower($charset) == 'utf-8')
 {
 while ($n < $length)
 {
 $currentByte = ord($string[$n] );
 if ($currentByte == 9 ||
 $currentByte == 10 ||
 (32 <= $currentByte && $currentByte <= 126)) // bbs.it-home.org
 {
 $ n++;
 $count++;
 } elseif (194 <= $currentByte && $currentByte <= 223)
 {
 $n += 2;
 $count += 2;
 } elseif (224 <= $currentByte && $currentByte <= 239)
 {
 $n += 3;
 $count += 2;
 } elseif (240 <= $currentByte && $currentByte <= 247)
 {
 $n += 4 ;
 $count += 2;
 } elseif (248 <= $currentByte && $currentByte <= 251)
 {
 $n += 5;
 $count += 2;
 } elseif ($currentByte == 252 || $currentByte == 253)
 {
 $n += 6;
 $count += 2;
 } else
 {
 $n++;
 $count++;
 }
 if ($count >= $length )
 {
 break;
 }
 }
 return $count;
 } else
 {
 for ($i = 0; $i < $length; $i++)
 {
 if (ord($string[$i ]) > 127)
 {
 $i++;
 $count++;
 }
 $count++;
 }
 return $count;
 }
}