Fatal error: Cannot re-assign $this in D:\php\function.php on line 40 这个解决方法，知道的请进

function.php
===============
<?php
function u2utf8($c) {
for($i = 0; $i < count($c); $i++)
$str = "";
if ($c < 0x80) {
$str .= $c;
}
else if ($c < 0x800) {
$str .= (0xC0 | $c >> 6);
$str .= (0x80 | $c & 0x3F);
}
else if ($c < 0x10000) {
$str .= (0xE0 | $c >> 12);
$str .= (0x80 | $c >> 6 & 0x3F);
$str .= (0x80 | $c & 0x3F);
}
else if ($c < 0x200000) {
$str .= (0xF0 | $c >> 18);
$str .= (0x80 | $c >> 12 & 0x3F);
$str .= (0x80 | $c >> 6 & 0x3F);
$str .= (0x80 | $c & 0x3F);
}
return $str;
}

function gb2utf8($gb) /* Program writen by sadly modified by agun */
{
if (!trim($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);

$ret = "";
$utf8 = "";
while ($gb) {
if (ord(substr($gb, 0, 1)) > 127) {
$this = substr($gb,0,2);
$gb = substr($gb, 2, strlen($gb));
$utf8 = u2utf8(hexdec($codetable[hexdec(bin2hex($this))-0x8080]));
for($i = 0; $i < strlen($utf8); $i += 3)
$ret .= chr(substr($utf8, $i, 3));
} else {
$ret .= substr($gb, 0, 1);
$gb = substr($gb, 1, strlen($gb));
}
}
return $ret;
}
?>
===============
test.php
===============
<?php
include("function.php");
$myfont = "湘潭大学374号信箱";
$my = gb2utf8($myfont);
Header("Content-type: image/png");
$im = ImageCreate(270, 50);
$black = ImageColorAllocate($im, 0, 0, 0);
$white = ImageColorAllocate($im, 184, 44, 6);
ImageTTFText($im, 20, 0, 4, 30, $white, "hy07.TTF", $my);
ImagePNG($im);
ImageDestroy($im);
?>