【本文原创，谢绝转载】

一、出现的问题

这几天做了手机上传照片并裁出缩略图的接口的测试，发现不管怎么，生成的缩略图都是一片漆黑。:-(

然后就把这个缩略图类单拿出来进行测试，发现只要是手机拍出来的照片都不能进行缩略图的处理。。。。

二、问题分析以及解决方案

经过群里的请教，发现问题可能是出现在文件的类型的判断上，因为png图片自带一个透明的图层，导致不能直接转换成jpg的文件，而手机排出的照片扩展名是jpg.

所以，得出的结论是手机拍出的是jpg扩展名的png图片。

由于扩展名是可以随意修改的，不是很能保证文件的信息的准确性，所以我们采用了 getimagesize 函数进行文件类型的获取。

//获取真实的图片类型
 list($width, $height, $type, $attr) = getimagesize($this->sur_file);
    switch($type) {
        case 1 :
            $img_type = 'gif';
            break;
        case 2 :
            $img_type = 'jpeg';
            break;
        case 3 :
            $img_type = 'png';
            break;
        case 15 :
            $img_type = 'wbmp';
            break;
        default :
            return false;
    }  

三、生成缩略图类

下面把修改后的生成缩略图的类贴出来，供大家指正~

/**
 * php生成缩略图类
 * 修改者 点点细雨
 * 文章出处 ： http://blog.csdn.net/diandianxiyu_geek
 * 2014-07-23 解决了图片类型不能正常识别的问题
 */
class thumb {

    public $sur_file; //读取的原图片
    public $des_file; //生成目标图片
    public $tem_file; //临时图片
    public $tag;  //缩略标签  0,默认,按固定的高宽生成  1,按比列或固定最大长度生成  -1,按某个宽度或某个高度缩小
    public $resize_width;  //$tag为0时,目标文件宽
    public $resize_height;  //$tag为0时,目标文件高
    public $sca_max; //$tag为1时,<0$sca_max<1时为缩小比例;$sca_max>1时为最大长度(高或宽之中的最大值)
    public $type;  //图片类型
    public $width;  //原图片宽
    public $height;  //原图片高
    public $size;     //原图大小

    //构造函数

    public function __construct($surpic, $reswid, $reshei, $despic, $mark, $scamax) {
        $this->sur_file = $surpic;
        $this->resize_width = $reswid;
        $this->resize_height = $reshei;
        $this->tag = $mark;
        $this->sca_max = $scamax;
        list($width, $height, $type, $attr) = getimagesize($this->sur_file);
        switch ($type) {
            case 1 :
                $img_type = 'gif';
                break;
            case 2 :
                $img_type = 'jpeg';
                break;
            case 3 :
                $img_type = 'png';
                break;
            case 15 :
                $img_type = 'wbmp';
                break;
            default :
                return false;
        }
        $this->type = $img_type; //获取图片类型
        $this->init_img(); //初始化图片
        $this->des_file = $despic; //目标图片地址
        $this->width = $width;
        $this->height = $height;
        $this->size = filesize($surpic);
        $this->new_img();
        imagedestroy($this->tem_file);
    }

    //图片初始化函数
    private function init_img() {
        if ($this->type == 'jpeg') {
            $this->tem_file = imagecreatefromjpeg($this->sur_file);
        } elseif ($this->type == 'jpg') {
            $this->tem_file = imagecreatefromjpeg($this->sur_file);
        } elseif ($this->type == 'gif') {
            $this->tem_file = imagecreatefromgif($this->sur_file);
        } elseif ($this->type == 'png') {
            $this->tem_file = imagecreatefrompng($this->sur_file);
        } elseif ($this->type == 'bmp') {
            $this->tem_file = imagecreatefrombmp($this->sur_file); //bmp.php中包含
        }
    }

    //图片生成函数
    private function new_img() {
        $ratio = ($this->width) / ($this->height); //原图比例
        $resize_ratio = ($this->resize_width) / ($this->resize_height); //缩略后比例
        $newimg = imagecreatetruecolor($this->resize_width, $this->resize_height); //生成新图片
        imagealphablending($newimg, false); //这里很重要,意思是不合并颜色,直接用$img图像颜色替换,包括透明色;
        imagesavealpha($newimg, true);
        if ($this->tag == 0) { //按固定高宽截取缩略图
            $newimg = imagecreatetruecolor($this->resize_width, $this->resize_height); //生成新图片
            if ($ratio >= $resize_ratio) {//即等比例下,缩略图的高比原图长,因此高不变
                imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, $this->resize_width, $this->resize_height, (($this->height) * $resize_ratio), $this->height);
            } elseif ($ratio < $resize_ratio) {//即等比例下,缩略图的宽比原图长,因此宽不变
                imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, $this->resize_width, $this->resize_height, $this->width, (($this->width) / $resize_ratio));
            }
        } elseif ($this->tag == 1) { //按固定比例或最大长度缩小
            if ($this->sca_max < 1) { //按比例缩小
                $newimg = imagecreatetruecolor((($this->width) * ($this->sca_max)), (($this->height) * ($this->sca_max))); //生成新图片
                imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, (($this->width) * ($this->sca_max)), (($this->height) * ($this->sca_max)), $this->width, $this->height);
            } elseif ($this->sca_max > 1) { //按某个最大长度缩小
                if ($ratio >= 1) { //宽比高长
                    $newimg = imagecreatetruecolor($this->sca_max, ($this->sca_max / $ratio)); //生成新图片
                    imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, $this->sca_max, ($this->sca_max / $ratio), $this->width, $this->height);
                } else {
                    $newimg = imagecreatetruecolor(($this->sca_max * $ratio), $this->sca_max); //生成新图片
                    imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, ($this->sca_max * $ratio), $this->sca_max, $this->width, $this->height);
                }
            }
        } elseif ($this->tag == -1) { //按某个宽度或某个高度缩小
            if ($resize_ratio >= 1) {//新高小于新宽,则图片按新宽度缩小
                $newimg = imagecreatetruecolor($this->resize_width, ($this->resize_width / $ratio)); //生成新图片
                imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, $this->resize_width, ($this->resize_width / $ratio), $this->width, $this->height);
            } elseif ($resize_ratio < 1) {//新宽小于新高,则图片按新高度缩小
                $newimg = imagecreatetruecolor(($this->resize_height * $ratio), $this->resize_height); //生成新图片
                imagecopyresampled($newimg, $this->tem_file, 0, 0, 0, 0, ($this->resize_height * $ratio), $this->resize_height, $this->width, $this->height);
            }
        }

        //输出新图片
        if ($this->type == 'jpeg' || $this->type == 'jpg') {
            imagejpeg($newimg, $this->des_file);
        } elseif ($this->type == 'gif') {
            imagegif($newimg, $this->des_file);
        } elseif ($this->type == 'png') {
            imagepng($newimg, $this->des_file);
        } elseif ($this->type == 'bmp') {
            imagebmp($newimg, $this->des_file); //bmp.php中包含
        }
    }

#function new_img() end
}