分析：
大概有2种情况，1是自身的页面编码，和获取的页面编码不一致。
另外的一种情况就是，目标页面用了gzip技术。
用以下的函数可解决这个问题：

<?php

function gzdecode($data) {
	$len = strlen ( $data );
	if ($len < 18 || strcmp ( substr ( $data, 0, 2 ), "\x1f\x8b" )) {
		return null; // Not GZIP format (See RFC 1952) 
	}
	$method = ord ( substr ( $data, 2, 1 ) ); // Compression method 
	$flags = ord ( substr ( $data, 3, 1 ) ); // Flags 
	if ($flags & 31 != $flags) {
		// Reserved bits are set -- NOT ALLOWED by RFC 1952 
		return null;
	}
	// NOTE: $mtime may be negative (PHP integer limitations) 
	$mtime = unpack ( "V", substr ( $data, 4, 4 ) );
	$mtime = $mtime [1];
	$xfl = substr ( $data, 8, 1 );
	$os = substr ( $data, 8, 1 );
	$headerlen = 10;
	$extralen = 0;
	$extra = "";
	if ($flags & 4) {
		// 2-byte length prefixed EXTRA data in header 
		if ($len - $headerlen - 2 < 8) {
			return false; // Invalid format 
		}
		$extralen = unpack ( "v", substr ( $data, 8, 2 ) );
		$extralen = $extralen [1];
		if ($len - $headerlen - 2 - $extralen < 8) {
			return false; // Invalid format 
		}
		$extra = substr ( $data, 10, $extralen );
		$headerlen += 2 + $extralen;
	}
	
	$filenamelen = 0;
	$filename = "";
	if ($flags & 8) {
		// C-style string file NAME data in header 
		if ($len - $headerlen - 1 < 8) {
			return false; // Invalid format 
		}
		$filenamelen = strpos ( substr ( $data, 8 + $extralen ), chr ( 0 ) );
		if ($filenamelen === false || $len - $headerlen - $filenamelen - 1 < 8) {
			return false; // Invalid format 
		}
		$filename = substr ( $data, $headerlen, $filenamelen );
		$headerlen += $filenamelen + 1;
	}
	
	$commentlen = 0;
	$comment = "";
	if ($flags & 16) {
		// C-style string COMMENT data in header 
		if ($len - $headerlen - 1 < 8) {
			return false; // Invalid format 
		}
		$commentlen = strpos ( substr ( $data, 8 + $extralen + $filenamelen ), chr ( 0 ) );
		if ($commentlen === false || $len - $headerlen - $commentlen - 1 < 8) {
			return false; // Invalid header format 
		}
		$comment = substr ( $data, $headerlen, $commentlen );
		$headerlen += $commentlen + 1;
	}
	
	$headercrc = "";
	if ($flags & 1) {
		// 2-bytes (lowest order) of CRC32 on header present 
		if ($len - $headerlen - 2 < 8) {
			return false; // Invalid format 
		}
		$calccrc = crc32 ( substr ( $data, 0, $headerlen ) ) & 0xffff;
		$headercrc = unpack ( "v", substr ( $data, $headerlen, 2 ) );
		$headercrc = $headercrc [1];
		if ($headercrc != $calccrc) {
			return false; // Bad header CRC 
		}
		$headerlen += 2;
	}
	
	// GZIP FOOTER - These be negative due to PHP's limitations 
	$datacrc = unpack ( "V", substr ( $data, - 8, 4 ) );
	$datacrc = $datacrc [1];
	$isize = unpack ( "V", substr ( $data, - 4 ) );
	$isize = $isize [1];
	
	// Perform the decompression: 
	$bodylen = $len - $headerlen - 8;
	if ($bodylen < 1) {
		// This should never happen - IMPLEMENTATION BUG! 
		return null;
	}
	$body = substr ( $data, $headerlen, $bodylen );
	$data = "";
	if ($bodylen > 0) {
		switch ($method) {
			case 8 :
				// Currently the only supported compression method: 
				$data = gzinflate ( $body );
				break;
			default :
				// Unknown compression method 
				return false;
		}
	} else {
	
		// I'm not sure if zero-byte body content is allowed. 
	// Allow it for now...  Do nothing... 
	}
	
	// Verifiy decompressed size and CRC32: 
	// NOTE: This may fail with large data sizes depending on how 
	//       PHP's integer limitations affect strlen() since $isize 
	//       may be negative for large sizes. 
	if ($isize != strlen ( $data ) || crc32 ( $data ) != $datacrc) {
		// Bad format!  Length or CRC doesn't match! 
		return false;
	}
	return $data;
}

?>