GD Library를 이용한 Thumbnail 생성 클래스입니다.
기본적인 이미지 리사이징 및 리사이징과 동시에 비율에 맞춰 Crop 하는 기능이 있고, 워터마크를 추가 할 수 있습니다. 본 코드는 PHP School의 Tip&Tech 게시판에 올라온 php사랑이님의 게시물을 기본 베이스로 하여 제가 필요한 부분을 일부 수정하였습니다.

원본글: [함수] GD를 이용한 기본기능 썸네일,CROP,CROP썸네일,텍스트 만들기

주된 수정 사항은 기존 워터마크에서 사용될 워터마크 이미지가 캔버스보다 크면 캔버스 이미지의 80%로 이미지를 축소하여 워터마크를 입히도록 변경하였습니다. 워터마크가 캔버스보다 작을 경우에는 그대로 반영이 되며, 워터마크가 찍히는 위치를 지정 할 수 있도록 변경하였습니다.

/** ======================================================
| @Author : 김종관
| @Email : apmsoft@gmail.com
| @HomePage : http://www.apmsoftax.com
| @Editor : Eclipse(default)
| @UPDATE : 2010-02-16
--------------------------------------------------------- */

# purpose : 이미지 효과주기
class ImageGD {
   private $filename;

   private $im;
   private $quality = 100;
   private $bgcolor = 0x7fffffff;
   private $fontsrc, $fontangle=0, $fontcolor = array(0,0,0), $fontsize = 20, $x=5, $y=5;

   # 시작
   public function __construct($filename=null){
      if(!file_exists($filename) && $filename)
      throw new Exception($filename);

      $this->filename = $filename;
   }

   # void 퀄리티 설정
   public function setCompressionQuality($quality){
      $this->quality = $quality;
   }

   # 칼라 채우기
   public function setFilledrectangle($image,$x1,$y1,$x2,$y2,$color){
      if(false === ($im = imagefilledrectangle($image,$x1,$y1,$x2,$y2,$color))) return false;
      return $im;
   }

   # 칼라 채우기 RGB
   public function setColorallocate($image,$r,$g,$b){
      if(0 > ($im = imagecolorallocate($image,$r,$g,$b))) return false;
      return $im;
   }

   # alpha
   public function setAlphablending($image,$boolean=false){
      imagealphablending($image, $boolean);
   }
   # alpha
   public function setSavealpha($image,$boolean=false){
      imagesavealpha($image, $boolean);
   }

   public function setFttext($image,$fontcolor,$text){
      imagefttext($image,$this->fontsize,$this->fontangle,$this->x,$this->y,$fontcolor,$this->fontsrc,$text);
   }

   # 폰트 파일 경로 지정
   public function setFont($fontsrc){ $this->fontsrc = $fontsrc; }

   # 칼라 지정
   public function setFontColor($color){ $this->color = $color; }

   # 폰트 사이즈
   public function setFontSize($pixel){ $this->fontsize = $pixel; }

   # 배경칼라
   public function setBgColor($bgcolor){ $this->bgcolor = $bgcolor; }

   # 폰트 앵글
   public function setFontAngle($angle){ $this->fontangle = $angle; }

   # x:y 축
   public function setXY($x,$y){ $this->x = $x; $this->y = $y; }

   # 텍스트 이미지 만들기
   public function writeTextImage($width,$height,$text){
      $this->im = self::createTrueImage($width,$height);
      self::setAlphablending($this->im);
      self::setFilledrectangle($this->im,0,0,$width,$height,$this->bgcolor);

      $fontcolor = self::setColorallocate($this->im,$this->fontcolor[0],$this->fontcolor[1],$this->fontcolor[2]);
      self::setFttext($this->im,$fontcolor,$text);
      self::setSavealpha($this->im,true);
   }

   public function setAntialias($image,$boolean=false){
      imageantialias($image,$boolean);
   }

   public function setTTFText($image,$size,$x,$y,$color,$text){
      imagettftext($image,$size,$this->fontangle,$x,$y,$color,$this->fontsrc,$text);
   }

   # 그림자 입체 텍스트 쓰기
   public function writeShadowText($width,$height,$text,$bgRGB=array(255,255,255),$mdRGB=array(128,128,128),$frontRGB=array(0,0,0))
   {
      $this->im = self::createTrueImage($width,$height);

      $bg = self::setColorallocate($this->im,$bgRGB[0],$bgRGB[1],$bgRGB[2]);
      $middle = self::setColorallocate($this->im, $mdRGB[0],$mdRGB[1],$mdRGB[2]);
      $front = self::setColorallocate($this->im, $frontRGB[0],$frontRGB[1],$frontRGB[2]);
      self::setFilledrectangle($this->im,0,0,$width-1,$height-1,$bg);

      // Add some shadow to the text
      self::setTTFText($this->im,$this->fontsize,$this->x,$this->y,$middle,$text);

      // Add the text
      self::setTTFText($this->im,$this->fontsize,$this->x - 1,$this->y - 1,$front,$text);
   }

   # 이미지 위에 텍스트 쓰기
   public function combineImageText($width,$height,$text,$filename=null){
      $this->im = self::createTrueImage($width,$height);
      self::setAntialias($this->im,true);
      $fontcolor = self::setColorallocate($this->im,$this->fontcolor[0],$this->fontcolor[1],$this->fontcolor[2]);

      $filename = ($filename) $filename : $this->filename;
      if(!$filename) throw new Exception(__CLASS__,':'.__METHOD__.':'.__LINE__);
      $image = self::readImage($filename);
      self::copy($this->im,$image,0,0,0,0,$width,$height);
      self::setTTFText($this->im,$this->fontsize,$this->x,$this->y,$fontcolor,$text);
   }

   # margin_r : 오른쪽 여백, margin_b : 아래여백
   # $locate : 1:좌측상단, 3:가운데, 5:우측하단, 0:오른쪽,하단마진 설정값
   public function filterWatermarks($marksfilename,$locate=3, $margin_r=10,$margin_b=10){
      if(!file_exists($marksfilename))
      throw new Exception(__CLASS__.':'.__METHOD__.':'.$marksfilename);

      $canvas = self::getImageSize($this->filename);
      $overlay = self::getImageSize($marksfilename);

      if($canvas->width < $overlay->width) {
         $overlay->width = ceil($canvas->width * 0.8);
         $overlay->height = ceil($canvas->height * 0.8);

         $thumbPath = pathinfo($marksfilename);
         $wmthumb = "{$thumbPath[dirname]}/{$thumbPath[filename]}_".$overlay->width."_".$overlay->height.".{$thumbPath[extension]}";

         if(!file_exists($wmthumb)) {
               $watermark = new ImageGD($marksfilename);
               $watermark->thumbnailImage($overlay->width, $overlay->height);
               $watermark->write($wmthumb);
         }

         $marksfilename = $wmthumb;
      }

      $this->im = self::readImage($this->filename);
      self::setAntialias($this->im,true);
      $image = self::readImage($marksfilename);

      $im_width = imagesx($this->im);
      $im_height = imagesy($this->im);

      $width = imagesx($image);
      $height = imagesy($image);
      switch($locate) {
         case 1 : // 좌측상단
         $im_x = 0;
         $im_y = 0;
         break;
         case 3 : // 가운데
         $im_x = ($im_width / 2) - ($width / 2);
         $im_y = ($im_height - $height) / 2;
         break;
         case 5 : // 우측 하단
         $im_x = $im_width - $width;
         $im_y = $im_height - $height;
         break;
         default :
         $im_x = $im_width - $width - $margin_r;
         $im_y = $im_height - $height - $margin_b;
      }

      self::copy($this->im,$image,$im_x,$im_y,0,0,$width,$height);
   }

   # void 이미지 자르기 int width,height,x,y
   public function cropImage($width,$height,$x,$y){
      $this->im = self::createTrueImage($width,$height);
      $image = self::readImage($this->filename);
      if(self::copy($this->im,$image,0,0,$x,$y,$width,$height) === false)
      throw new Exception(__METHOD__);
   }

   # void 이미지 자르기 (center) int width,height
   public function cropThumbnailImage($width,$height){
      $imgsize = self::getImageSize($this->filename);

      # 조정
      $im_x = 0;
      $im_y = 0;
      $image_x = 0;
      $image_y = 0;

      $wm = $imgsize->width/$width;
      $hm = $imgsize->height/$height;
      $h_height = $height/2;
      $w_height = $width/2;

      if($imgsize->width > $imgsize->height){
      $width = $imgsize->width / $hm;
      $half_width = $width / 2;
      $im_x = -($half_width - $w_height);
      }else if(($imgsize->width <$imgsize->height) || ($imgsize->wdith == $imgsize->height)){
      $height = $imgsize->height / $wm;
      $half_height = $height / 2;
      $im_y = $half_height - $h_height;
      }

      $this->im = self::createTrueImage($width,$height);
      $image = self::readImage($this->filename);
      if(self::copyResampled($this->im,$image,$im_x,$im_y,$image_x,$image_y,$width,$height,$imgsize->width,$imgsize->height) === false)
      throw new Exception(__METHOD__);

      return true;
   }

   # 썸네일 이미지 만들기 int width, height
   public function thumbnailImage($width,$height){
      $imgsize = self::getImageSize($this->filename);

      # 썸네일 사진 사이즈 설정
      if($imgsize->width>$imgsize->height){
         $height= ceil(($imgsize->height*$width)/$imgsize->width);
      }
      else if($imgsize->width<$imgsize->height || $imgsize->width == $imgsize->height){
         $width= ceil(($imgsize->width*$height)/$imgsize->height);
      }

      $this->im = self::createTrueImage($width,$height);
      $image = self::readImage($this->filename);
      self::setAlphablending($this->im, false);
      if(self::copyResampled($this->im, $image, 0,0,0,0,$width,$height,$imgsize->width,$imgsize->height) ===false)
      throw new Exception(__METHOD__);
      return true;
   }

   # imagecopy
   public function copy($im,$image,$im_x,$im_y,$image_x,$image_y,$width,$height){
      if(imagecopy($im,$image,$im_x,$im_y,$image_x,$image_y,$width,$height) === false) return false;
      return true;
   }

   # imagemerge
   public function copyMerge($im,$image,$im_x,$im_y,$image_x,$image_y,$width,$height,$pct){
      if(!imagecopymerge($im,$image,$im_x,$im_y,$image_x,$image_y,$width,$height,$pct)) return false;
      return true;
   }

   # imagecopyresampled
   public function copyResampled($im,$image,$im_x,$im_y,$image_x,$image_y,$width,$height,$oriwidth,$oriheight){
      //self::setSavealpha($im,true);
      if(imagecopyresampled($im,$image,$im_x,$im_y,$image_x,$image_y,$width,$height,$oriwidth,$oriheight) ===false) return false;
      return true;
   }

   # void : createtruecolor
   public function createTrueImage($width,$height){
      return $im = imagecreatetruecolor($width,$height);
   }

   # void
   public function readImage($filename){
      $count = strrpos($filename,'.');
      $extention = strtolower(substr($filename, $count+1));
         switch($extention){
         case 'gif': $image = imagecreatefromgif($filename); break;
         case 'png': $image = imagecreatefrompng($filename); break;
         case 'jpeg':
         case 'jpg': $image = imagecreatefromjpeg($filename); break;
         default : throw new Exception('i can\'t the image format');
      }
      return $image;
   }

   # string filename
   public function write($filename){
      $count = strrpos($filename,'.');
      $extention = strtolower(substr($filename, $count+1));
      switch($extention){
         case 'gif': imagegif($this->im,$filename); return true; break;
         case 'png':
         self::setSavealpha($this->im, true);
         imagepng($this->im,$filename,($this->quality/10)-1); return true; break;
         case 'jpg':
         case 'jpeg': imagejpeg($this->im,$filename,$this->quality); return true; break;
         default : return false;
      }
   }

   # @ void : GD 버전
   public function getVersion(){
      if(function_exists('gd_info')){
         $info = gd_info();
         return preg_replace('/bundled \((.*) compatible\)/','\\1', $info['GD Version']);
      }
      return false;
   }

   # 이미지 사이즈
   public function getImageSize($filename=null){
      $filename = ($filename) $filename : $this->filename;
      $img_info = getImageSize($filename);
      return json_decode(json_encode(array('width'=>$img_info[0],'height'=>$img_info[1],'mime'=>$img_info['mime'])));
   }

   # @ void
   public function destroy(){
      if(is_resource($this->im)) imagedestroy($this->im);
   }

   public function __destruct(){
      self::destroy();
   }
}

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