生成图像缩略图（支持：JPEG，GIT，PNG，BMP）

<?php
class Thumb
{

 public function create($srcPath, $dstPath, $dstWidth, $dstHeight)
{
 if (!file_exists($srcPath)) {
 return false;
}

 @$srcSize = getimagesize($srcPath);
 if (empty($srcSize)) {
 return false;
}

 $srcWith = intval($srcSize[0]);
 $srcHeight = intval($srcSize[1]);

//如果原始图片的尺寸大于指定缩略图的尺寸，则生成缩略图，否则拷贝原始文件
 if ($srcWith <= $dstWidth && $srcHeight <= $dstHeight) {
 return copy($srcPath, $dstPath);
}

//读取原始图片资源
 @$srcImage = imagecreatefromjpeg($srcPath);
 if (empty($srcImage)) {
 @$srcImage = imagecreatefromgif($srcPath);
}
 if (empty($srcImage)) {
 @$srcImage = imagecreatefrompng($srcPath);
}
 if (empty($srcImage)) {
 @$srcImage = $this->_imageCreateFromBMP($srcPath);
}

 if (empty($srcImage)) {
 return false;
}

//获取缩略图的尺寸，并据此生成新的图像
 $dstSize = $this->_getDstSize(
 $srcWith, $srcHeight, $dstWidth, $dstHeight
);

 @$dstImage = imagecreatetruecolor(
 $dstSize['width'], $dstSize['height']
);

@imagecopyresampled(
 $dstImage, $srcImage , 0, 0, 0, 0,
 $dstSize['width'], $dstSize['height'],
 $srcWith, $srcHeight
);

 return @imagepng($srcPath, $dstPath);
}

 private function _imageCreateFromBMP($filePath)
{
 $fileHandle = fopen($filePath, 'rb');
 if (empty($fileHandle)) {
 return false;
}

 $file = unpack(
'vfile_type/Vfile_size/Vreserved/Vbitmap_offset',
 fread($fileHandle, 14)
);

 if ($file['file_type'] != 19778) {
 return false;
}

 $bmp = unpack(
'Vheader_size/Vwidth/Vheight/vplanes/'.
'vbits_per_pixel/Vcompression/Vsize_bitmap/'.
'Vhoriz_resolution/Vvert_resolution/Vcolors_used/Vcolors_important',
 fread($fileHandle, 40)
);
 $bmp['colors'] = pow(2, $bmp['bits_per_pixel']);
 if ($bmp['size_bitmap'] == 0) {
 $bmp['size_bitmap'] = $file['file_size'] - $file['bitmap_offset'];
}
 $bmp['bytes_per_pixel'] = $bmp['bits_per_pixel'] / 8;
 $bmp['bytes_per_pixel2'] = ceil($bmp['bytes_per_pixel']);
 $bmp['decal'] = $bmp['width'] * $bmp['bytes_per_pixel'] / 4;
 $bmp['decal'] -= floor($bmp['width'] * $bmp['bytes_per_pixel'] / 4);
 $bmp['decal'] = 4 - (4 * $bmp['decal']);
 if ($bmp['decal'] == 4) {
 $bmp['decal'] = 0;
}

 $palette = array();
 if ($bmp['colors'] < 16777216) {
 $palette = unpack(
 'V' . $bmp['colors'],
 fread($fileHandle, $bmp['colors'] * 4)
);
}
 $image = fread($fileHandle, $bmp['size_bitmap']);
 $vide = chr(0);
 $res = imagecreatetruecolor($bmp['width'], $bmp['height']);
 $p = 0;

 $y = $bmp['height'] - 1;
 while ($y >= 0) {
 $x = 0;
 while ($x < $bmp['width']) {
 if ($bmp['bits_per_pixel'] == 24) {
 $color = unpack('V', substr($image, $p, 3) . $vide);
 } else if ($bmp['bits_per_pixel'] == 16) {
 $color = unpack('n', substr($image, $p, 2));
 $color[1] = $palette[$color[1]+1];
 } else if ($bmp['bits_per_pixel'] == 8) {
 $color = unpack('n', $vide . substr ($image, $p, 1));
 $color[1] = $palette[$color[1]+1];
 } else if ($bmp['bits_per_pixel'] ==4) {
 $color = unpack('n', $vide . substr($image, floor($p), 1));
 if (($p * 2) % 2 == 0) {
 $color[1] = ($color[1] >> 4);
 } else {
 $color[1] = ($color[1] & 0x0F);
}
 $color[1] = $palette[$color[1] + 1];
 } else if ($bmp['bits_per_pixel'] == 1) {
 $color = unpack('n', $vide . substr($image, floor($p), 1));
 switch (($p * 8) % 8) {
 case 0:
 $color[1] = ($color[1] >> 7);
break;
 case 1:
 $color[1] = ($color[1] & 0x40) >> 6;
break;
 case 2:
 $color[1] = ($color[1] & 0x20) >> 5;
break;
 case 3:
 $color[1] = ($color[1] & 0x10) >> 4;
break;
 case 4:
 $color[1] = ($color[1] & 0x8) >> 3;
break;
 case 5:
 $color[1] = ($color[1] & 0x4) >> 2;
break;
 case 6:
 $color[1] = ($color[1] & 0x2) >> 1;
break;
 case 7:
 $color[1] = ($color[1] & 0x1);
break;
}
 $color[1] = $palette[$color[1] + 1];
 } else {
 return false;
}
 imagesetpixel($res, $x, $y, $color[1]);
$x++;
 $p += $bmp['bytes_per_pixel'];
}
$y--;
 $p += $bmp['decal'];
}
fclose($fileHandle);
 return $res;
}

 private function _getDstSize($srcWith, $srcHeight, $dstWidth, $dstHeight)
{
 $size = array('width' => $srcWith, 'height' => $srcHeight);
 if ($dstWidth > 0 && $dstHeight > 0) {
 if ($srcWith > 0 && $srcHeight > 0) {
 if ($srcWith / $srcHeight >= $dstWidth / $dstHeight) {
 if ($srcWith > $dstWidth) {
 $size['width'] = $dstWidth;
 $size['height'] = $srcHeight * $dstWidth / $srcWith;
}
 } else {
 if ($srcHeight > $dstHeight) {
 $size['width'] = $srcWith * $dstHeight / $srcHeight;
 $size['height'] = $dstHeight;
}
}
}
}
 return $size;
}
}