Question about GBK string truncation ~

Question about GBK string truncation ~~~~ This is a function for intercepting GBK strings: PHPcodefunctiongb_substr ($ str, $ len) {$ count = 0; for ($ I = 0) question about intercepting GBK strings ~~~~
This is a function that intercepts GBK strings:

PHP code

  function  gb_substr( $str ,  $len ){              $count  = 0;              for ( $i =0;  $i < strlen ( $str );  $i ++){                  if ( $count  ==  $len )  break ;                  if (preg_match( "/[\x80-\xff]/" ,  substr ( $str ,  $i , 1))){        ++ $i ;          }                  ++ $count ;                      }              return   substr ( $str , 0,  $i );        }

My question is, is each GBK and GB2312 not necessarily two bytes? Is the length of X 2 enough to be intercepted?
For example, I want to take 3 characters: 3*2 = 6, that is, I want to take 6 characters.

Is that true?

------ Solution --------------------
Using functions of the mb series, GBK pairs are non-ASCII characters, and ASCII is still 1 byte.
------ Solution --------------------
No, because it may contain ascii numbers or characters.

Discussion
This is a function that intercepts GBK strings:
PHP code

Function gb_substr ($ str, $ len ){
$ Count = 0;
For ($ I = 0; $ I <strlen ($ str); $ I ++ ){
If ($ count = $ len) break ......