/**
 * 生成图片
 * @param string $im 源图片路径
 * @param string $dest 目标图片路径
 * @param int $maxwidth 生成图片宽
 * @param int $maxheight 生成图片高
 */
function resizeImage($im, $dest, $maxwidth, $maxheight) {
 $img = getimagesize($im);
 switch ($img[2]) {
  case 1:
   $im = @imagecreatefromgif($im);
   break;
  case 2:
   $im = @imagecreatefromjpeg($im);
   break;
  case 3:
   $im = @imagecreatefrompng($im);
   break;
 }
  
 $pic_width = imagesx($im);
 $pic_height = imagesy($im);
 $resizewidth_tag = false;
 $resizeheight_tag = false;
 if (($maxwidth && $pic_width > $maxwidth) || ($maxheight && $pic_height > $maxheight)) {
  if ($maxwidth && $pic_width > $maxwidth) {
   $widthratio = $maxwidth / $pic_width;
   $resizewidth_tag = true;
  }
  
  if ($maxheight && $pic_height > $maxheight) {
   $heightratio = $maxheight / $pic_height;
   $resizeheight_tag = true;
  }
  
  if ($resizewidth_tag && $resizeheight_tag) {
   if ($widthratio < $heightratio)
    $ratio = $widthratio;
   else
    $ratio = $heightratio;
  }
  
  
  if ($resizewidth_tag && !$resizeheight_tag)
   $ratio = $widthratio;
  if ($resizeheight_tag && !$resizewidth_tag)
   $ratio = $heightratio;
  $newwidth = $pic_width * $ratio;
  $newheight = $pic_height * $ratio;
  
  if (function_exists("imagecopyresampled")) {
   $newim = imagecreatetruecolor($newwidth, $newheight);
   imagecopyresampled($newim, $im, 0, 0, 0, 0, $newwidth, $newheight, $pic_width, $pic_height);
  } else {
   $newim = imagecreate($newwidth, $newheight);
   imagecopyresized($newim, $im, 0, 0, 0, 0, $newwidth, $newheight, $pic_width, $pic_height);
  }
  
  imagejpeg($newim, $dest);
  imagedestroy($newim);
 } else {
  imagejpeg($im, $dest);
 }
}
  
/**
 * 图片压缩处理
 * @param string $sFile 源图片路径
 * @param int $iWidth 自定义图片宽度
 * @param int $iHeight 自定义图片高度
 * @return string  压缩后的图片路径
 */
function getThumb($sFile,$iWidth,$iHeight){
 //图片公共路径
 $public_path = '';
 //判断该图片是否存在
 if(!file_exists($public_path.$sFile)) return $sFile;
 //判断图片格式(图片文件后缀)
 $extend = explode("." , $sFile);
 $attach_fileext = strtolower($extend[count($extend) - 1]);
 if (!in_array($attach_fileext, array('jpg','png','jpeg'))){
  return '';
 }
 //压缩图片文件名称
 $sFileNameS = str_replace(".".$attach_fileext, "_".$iWidth.'_'.$iHeight.'.'.$attach_fileext, $sFile);
 //判断是否已压缩图片，若是则返回压缩图片路径
 if(file_exists($public_path.$sFileNameS)){
  return $sFileNameS;
 }
  
 //生成压缩图片，并存储到原图同路径下
 resizeImage($public_path.$sFile, $public_path.$sFileNameS, $iWidth, $iHeight);
 if(!file_exists($public_path.$sFileNameS)){
  return $sFile;
 }
 return $sFileNameS;
}

//原图 img/img.jpg
//生成压缩图 img/img_300_300.jpg
getThumb('img/img.jpg',300,300);