The most accurate php function to intercept string length_php tips

It is said to be the most accurate interception length. In fact, I am not sure whether it is the most accurate. Just look at the effect below to know how accurate it is:
First, the string for testing:

<&#63;php 
header("Content-Type:text/html;charset=utf-8"); 
echo cn_substr_utf8('我是一个，和哈，哦也，，国家！',12); 
echo '<br />',cn_substr_utf8('ai\'2145m a ch3我[是一，个，和哈，哦也，，国家！',12); 
echo '<br />',cn_substr_utf8('【我，是一，个，和哈，哦也，，国家！',12); 
echo '<br />',cn_substr_utf8('我是一，个，和哈，哦也，，国家！',12); 
echo '<br />',cn_substr_utf8('我是,一，个，和哈，哦也，，国家！',12); 
echo '<br />',cn_substr_utf8('我,是,一，个，和哈，哦也，，国家！',12); 
echo '<br />',cn_substr_utf8('我是asd一，个，和哈，哦也，，国家！',12); 
echo '<br />',cn_substr_utf8('【我i\'m[是一，个，和哈，哦也，，国家！',12); 
echo '<br />',cn_substr_utf8('【i\'m a ch我[是一，个，和哈，哦也，，国家！',12); 
echo '<br />',cn_substr_utf8('【i\'2145m a ch3我[是一，个，和哈，哦也，，国家！',12); 

The following is the rendering of accurately intercepting the string:

The specific function code is as follows:

//utf-8中文截取，单字节截取模式 
function cn_substr_utf8($str,$length,$append='...',$start=0){ 
  if(strlen($str)<$start+1){ 
    return ''; 
  } 
  preg_match_all("/./su",$str,$ar); 
  $str2=''; 
  $tstr=''; 
  //www.phpernote.com 
  for($i=0;isset($ar[0][$i]);$i++){ 
    if(strlen($tstr)<$start){ 
      $tstr.=$ar[0][$i]; 
    }else{ 
      if(strlen($str2)<$length + strlen($ar[0][$i])){ 
        $str2.=$ar[0][$i]; 
      }else{ 
        break; 
      } 
    } 
  } 
  return $str==$str2&#63;$str2:$str2.$append; 
} 

If you think it is not accurate enough, you can make improvements or innovate on this basis. I hope this article about the PHP interception string length function will be helpful to your study.