php 中文字符串截取子串

function string_substring($string,$start,$length)
{
$countstart=0;
$countlength=0;
$printstring="";
for($i=0;$i<strlen($string);$i++)
{
while($countstart<$start)
{
$countstart++;
if(ord(substr($string,$i,1))>128)
{
$i+=3;
}
else
{
$i++;
}
}
while($countlength<$length)
{
$countlength++;
if(ord(substr($string,$i,1))>128)
{
$printstring.=substr($string,$i,3);
$i+=3;
}
else
{
$printstring.=substr($string,$i,1);
$i++;
}
}
}
return $printstring;
}