<?php
function ImageToJPG($srcFile,$dstFile,$towidth,$toheight)
{
  $quality=80;
  $data = @GetImageSize($srcFile);
  switch ($data['2'])
  {
  case 1:
    $im = imagecreatefromgif($srcFile);
    break;
  case 2:
    $im = imagecreatefromjpeg($srcFile);
    break;
  case 3:
    $im = imagecreatefrompng($srcFile);
    break;
  case 6:
  $im = ImageCreateFromBMP( $srcFile );
  break;
  }
  // $dstX=$srcW=@ImageSX($im);
  // $dstY=$srcH=@ImageSY($im);
  $srcW=@ImageSX($im);
  $srcH=@ImageSY($im);
  //$towidth,$toheight
  if($toheight/$srcW > $towidth/$srcH){
    $b = $toheight/$srcH;
  }else{
    $b = $towidth/$srcW;
  }
  //計算出圖片縮放后的寬高
  // floor 舍去小數點部分，取整
  $new_w = floor($srcW*$b);
  $new_h = floor($srcH*$b);
  $dstX=$new_w;
  $dstY=$new_h;
  $ni=@imageCreateTrueColor($dstX,$dstY);
  @ImageCopyResampled($ni,$im,0,0,0,0,$dstX,$dstY,$srcW,$srcH);
  @ImageJpeg($ni,$dstFile,$quality);
  @imagedestroy($im);
  @imagedestroy($ni);
}
//ImageToJPG('源文件名','目標文件名',目標寬,目標高);
ImageToJPG('test2.png','test2.jpg',80,50);