php 等比例缩略图

function ResizeImage($oldpic,$newpic,$maxwidth=800,$maxheight=600){
    if( eregi('.jpg',$oldpic) ){
         $im = imagecreatefromjpeg($oldpic);
    }elseif( eregi('.png',$oldpic) ){
         $im = imagecreatefrompng($oldpic);
    }elseif( eregi('.gif',$oldpic) ){
        $im = imagecreatefromgif($oldpic);
    }else{
        //die("图片格式不对$oldpic");
    }
    $width = imagesx($im);
    $height = imagesy($im);
    if(($maxwidth && $width > $maxwidth) || ($maxheight && $height > $maxheight)){
        if($maxwidth && $width > $maxwidth){
            $widthratio = $maxwidth/$width;
            $RESIZEWIDTH=true;
        }
        if($maxheight && $height > $maxheight){
            $heightratio = $maxheight/$height;
            $RESIZEHEIGHT=true;
        }
        if($RESIZEWIDTH && $RESIZEHEIGHT){
            if($widthratio < $heightratio){
                $ratio = $widthratio;
            }else{
                $ratio = $heightratio;
            }
        }elseif($RESIZEWIDTH){
            $ratio = $widthratio;
        }elseif($RESIZEHEIGHT){
            $ratio = $heightratio;
        }
        $newwidth = $width * $ratio;
        $newheight = $height * $ratio;
        if(function_exists("imagecopyresampled")){
              $newim = imagecreatetruecolor($newwidth, $newheight);
              imagecopyresampled($newim, $im, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
        }else{
            $newim = imagecreate($newwidth, $newheight);
            //imagepstext($newim,'tryr','10','12','000000','000000',0,0,5,5,1,1);
            imagecopyresized($newim, $im, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
        }
        ImageJpeg ($newim,$newpic);
        ImageDestroy ($newim);
    }else{
        ImageJpeg ($im,$newpic);
    }
    ImageDestroy ($im);
}