PHP utf-8中文截取无乱码(字符串判断版本）

/*E4 B8 AD E5 9B BD   中国
1110 0100    10111000   10101101
1110 0101    1001101110111101

61  a
0011 1101

0xxx xxxx ,1个字节
110xx xxxx , 2个字节
1110 xxxx, 3
1111 0xxxx 4...
*/<pre name="code" class="php">$str = '中华人aaa民共b和国,c万岁';

function utf8sub($str,$len){
if($len <=0) return '';
$length = strlen($str);
$offset=0;
$chars=0;
$res='';
while($chars<$len && $offset<$length){
$heigh = decbin(ord(substr($str, $offset,1)));
echo $heigh ,'</br>';
if(strlen($heigh)<8){
$count=1;
}else if(substr($heigh,0,3) == '110'){
$count=2;
}else if(substr($heigh,0,4) == '1110'){
$count=3;
}else if(substr($heigh,0,5) == '11110'){
$count=4;
}else if(substr($heigh,0,6) == '111110'){
$count=5;
}else if(substr($heigh,0,7) == '1111110'){
$count=6;
}
$res .= substr($str, $offset,$count);
$chars += 1;
$offset += $count;
}
return $res;
}

echo utf8sub($str,10);