php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;
    }
}

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