// 目的地尺寸及檔案
$size = "326x228";
$output_file = "thumb.jpg";
 
list($resizeW, $resizeH) = split("x", $size);
 
// 先抓出圖片的=長寬資訊
@list($width, $height, $type, $attr) = getimagesize( $source_img );
 
// 確認是否有抓到正確的檔案
if($resizeW!=0 && $resizeH!=0)
{
 
    // 找出正確的輸出比例
    $aspect_ratio =  $resizeW / $resizeH;
    $factor = (($width/$height) > $aspect_ratio)?($width/$resizeW):($height/$resizeH);
     
    $newWidth = round($width / $factor);
    $newHeight = round($height / $factor);
     
    $offset_w = round(abs( $newWidth - $resizeW ) / 2);
    $offset_h = round(abs( $newHeight - $resizeH ) / 2);
     
    $offset_w = 0;
    $offset_h = 0;
     
    $cmd = 'convert '.$source_img.' -thumbnail '.$size.'^ -crop '.$size.'+'.$offset_w.'+'.$offset_h.' '.$output_file; 
 
    pclose(popen("$cmd ", 'r'));
}