（在线等）这里什么问题？Fatal error: Cannot re-assign $this in e:\APP\www\ZSSL\www\trs\stat\gb2utf8.php on line 20

Question

//是函数出现问题，还是配置没有配好？？
function gb2utf8($gb)
{
if(!trim($gb))
return $gb;
$filename="gb2312.txt";
$tmp=file($filename);
$codetable=array();
while(list($key,$value)=each($tmp))
$codetable[hexdec(substr($value,0,6))]=substr($value,7,6);$utf8="";
while($gb)
{
if (ord(substr($gb,0,1))>127)
{
$this=substr($gb,0,2);     //line20
$gb=substr($gb,2,strlen($gb));
$utf8.=u2utf8(hexdec($codetable[hexdec(bin2hex($this))-0x8080]));
}
else
{
$gb=substr($gb,1,strlen($gb));
$utf8.=u2utf8(substr($gb,0,1));
}
}$ret="";
for($i=0;$i<strlen($utf8);$i+=3)
$ret.=chr(substr($utf8,$i,3));return $ret;
}