http://www.devx.com/webdev/Article/37179/0/page/6
<?php
// dark_filter.php
  $im = imagecreatefromjpeg("tree.jpg");
   $size = getimagesize("tree.jpg");
   $L=$size[0];
   $H=$size[1];
   
   for($j=0;$j<$H;$j++){
      for($i=0;$i<$L;$i++){
         $rgb = imagecolorat($im, $i, $j);
         $r = $rgb&0x00FF0000;    
         $r = $r >> 16;
         $g = $rgb&0x0000FF00; 
         $g = $g >> 8;
         $b = $rgb&0x0000FF;
   
         $frac = 0.5; // 0.0 < frac < 1.0
   
         $rr = (int)($r*$frac);
         $gg = (int)($g*$frac);
         $bb = (int)($b*$frac);
                 
         $result = (0x000000FF<<24)|
            ($rr <<16)|($gg << 8)|$bb;
   
         $new_r = ($result >> 16) & 0xFF;
         $new_g = ($result >> 8) & 0xFF;
         $new_b = $result & 0xFF;
   
         $new_color = imagecolorallocate(
            $im, $new_r, $new_g, $new_b); 
         imagesetpixel($im, $i, $j, $new_color);
      }
   } 
   header("Content-type: image/jpeg");
   imagejpeg($im);
   ?>

   <?php
/*
This application uses a filter that increases the black color in an image by choosing a random threshold value between 0 and 255, and then setting all the pixels where the R, G, and B values exceed that threshold to black (0, 0, 0). Note that this operates primarily on the darker-colored pixels in the image 
*/
   $im = imagecreatefromjpeg("tree.jpg");
   $size = getimagesize("tree.jpg");
   $L=$size[0];
   $H=$size[1];
   $k=1;
   for($j=0;$j<$H;$j++){
      for($i=0;$i<$L;$i++){
         $rgb = imagecolorat($im, $i, $j);
         $r = ($rgb >> 16) & 0xFF;
         $g = ($rgb >> 8) & 0xFF;
         $b = $rgb & 0xFF;
   
   
         $profile = rand(0,255);
         if(($r<$profile)&&($g<$profile)&&
            ($b<$profile))
         {
            $new_color = imagecolorallocate(
                $im, 0, 0, 0); 
            imagesetpixel($im, $i, $j, $new_color);
         }
      }
   }    
   header("Content-type: image/jpeg");
   imagejpeg($im);
   ?>
   
   

   <?php
/*
The reverse of a black filter takes dark pixels and changes them to another color—white in this example—creating a "snow" filter 
*/ 
  $im = imagecreatefromjpeg("tree.jpg");
   $size = getimagesize("tree.jpg");
   $L=$size[0];
   $H=$size[1];
   
   for($j=0;$j<$H;$j++){
      for($i=0;$i<$L;$i++){
     
         $rgb = imagecolorat($im, $i, $j); 
   
         $red = 255;
         $green = 255;
         $blue = 255;
   
         $r = ($rgb >> 16) & 0xFF;
         $g = ($rgb >> 8) & 0xFF;
         $b = $rgb & 0xFF;
      
         if(($r<50)&&($g<50)&&($b<50)) {
            $color = imagecolorallocate(
               $im, $red, $green, $blue); 
            imagesetpixel($im, $i, $j, $color);
         }
      }
   }
   header("Content-type: image/jpeg");
   imagejpeg($im);
   ?>

   <?php
/*
You can display an image in shades of a specific color using this filter by AND'ing the existing color with the filter-specific color. The example in Figure 12 uses a hard-coded orange hue, but you can use any color you like.

*/
   $im = imagecreatefromjpeg("tree.jpg");
   $size = getimagesize("tree.jpg");
   $L=$size[0];
   $H=$size[1];
   
   
   for($j=0;$j<$H;$j++){
      for($i=0;$i<$L;$i++){
     
         $rgb = imagecolorat($im, $i, $j);
   
         $red = 255;
         $green = 124;
         $blue = 28;
   
         $r = ($rgb >> 16) & 0xFF;
         $g = ($rgb >> 16) & 0xFF;
         $b = $rgb & 0xFF;
      
         $color = imagecolorallocate(
             $im, $red, $green, $blue);
          
         $result = $rgb & $color;
   
         imagesetpixel($im, $i, $j, $result); 
      }
   } 
   header("Content-type: image/jpeg");
   imagejpeg($im);
   ?>
   <?php
//Listing paint_filter.php
   $im = imagecreatefromjpeg("tree.jpg");
   $size = getimagesize("tree.jpg");
   $L=$size[0];
   $H=$size[1]; 
   
   for($m=0;$m<$H;$m++){
      for($i=0;$i<$L;$i++){
     
         $rgb = imagecolorat($im, $i, $m);
   
         $red = ($rgb >> 16) & 0xFF;
         $green = ($rgb >> 16) & 0xFF;
         $blue = $rgb & 0xFF;
      
         $tR = $red;
         $tG = $green;
         $tB = $blue;
   
   
         $R = array(255,255,0,0,0,
             255,255,0,64,128,192,255,255,-1);
         $G = array(0,255,0,255,0,255,
             0,255,64,128,192,200,175,-1);
         $B = array(0,255,0,0,255,0,
             255,255,64,128,192,0,175,-1);
   
         $R[13]=$red;
         $G[13]=$green;
         $B[13]=$blue;
   
        for($j=0;$j<13;$j++){
            for($k=$j+1;$k<14;$k++)
            {
               if($R[$j]<$R[$k]){
                  $aux=$R[$j];$R[$j]=$R[$k];$R[$k]=$aux;}
               if($G[$j]<$G[$k]){
                  $aux=$G[$j];$G[$j]=$G[$k];$G[$k]=$aux;}
               if($B[$j]<$B[$k]){
                  $aux=$B[$j];$B[$j]=$B[$k];$B[$k]=$aux;}
            }
         }
   
     
     for($j=0;$j<14;$j++){ 
         if(($R[$j]==$tR)&&($j==0)){$xR=$R[$j+1];break;}
         if(($R[$j]==$tR)&&($j==8)){$xR=$R[$j-1];break;}
         if(($R[$j]==$tR)&&($j!=0)&&($j!=8)){
            $xR=min(($tR-$R[$j-1]),($R[$j+1]-$tR));
            break;
        }
     }
      
   
      for($j=0;$j<14;$j++){   
         if(($G[$j]==$tG)&&($j==0)){$xG=$G[$j+1];break;}
         if(($G[$j]==$tG)&&($j==8)){$xG=$G[$j-1];break;}
         if(($G[$j]==$tG)&&($j!=0)&&($j!=8)){
            $xG=min(($tG-$G[$j-1]),($G[$j+1]-$tG));
            break;
         } 
      }
   
      for($j=0;$j<14;$j++){ 
         if(($B[$j]==$tB)&&($j==0)){$xB=$B[$j+1];break;}
         if(($B[$j]==$tB)&&($j==8)){$xB=$B[$j-1];break;}
         if(($B[$j]==$tB)&&($j!=0)&&($j!=8)){
            $xB=min(($tB-$B[$j-1]),($B[$j+1]-$tB));
            break;
         }
      }     
              
   $color = imagecolorallocate(
       $im, abs($xR), abs($xG), abs($xB));
   imagesetpixel($im, $i, $m, $color);  
    }
   }
      
   header("Content-type: image/jpeg");
   imagejpeg($im);  
   ?> 
   <?php
//brightness.php
   $im = imagecreatefromjpeg("tree.jpg");
   $size = getimagesize("tree.jpg");
   $L=$size[0];
   $H=$size[1]; 
   
   for($j=0;$j<$H;$j++){
      for($i=0;$i<$L;$i++){  
         $rgb = imagecolorat($im, $i, $j); 
   
         $r = ($rgb >> 16) & 0xFF;
         $g = ($rgb >> 8) & 0xFF;
         $b = $rgb & 0xFF;
   
         $getArrayHSB = RGBtoHSB($r,$g,$b);
   
         $brightness = 1.5; //[0.0, 1.0] - darker,  
             [1.0, 10.0] - brightner
         $getArrayHSB[2] = $brightness*$getArrayHSB[2];
         $getArrayHSB[2] = max(0.0,min($getArrayHSB[2],255.0)); 
        
         $getArrayRGB = HSBtoRGB(
            $getArrayHSB[0], $getArrayHSB[1], 
            $getArrayHSB[2]);
   
         $hsbrgb = imagecolorallocate(
            $im, $getArrayRGB[0], $getArrayRGB[1], 
            $getArrayRGB[2]);
         $result = (($rgb & 0xff000000)|($hsbrgb));
    
         imagesetpixel($im, $i, $j, $result);    
      }
   }   
   header("Content-type: image/jpeg");
   imagejpeg($im);  
      
   ?>

   <?php
//Modifying the saturation of an image 
//changes the amount of color in its pixels. 
  $im = imagecreatefromjpeg("tree.jpg");
   $size = getimagesize("tree.jpg");
   $L=$size[0];
   $H=$size[1]; 
   
   for($j=0;$j<$H;$j++){
      for($i=0;$i<$L;$i++){ 
         $rgb = imagecolorat($im, $i, $j); 
      
         $r = ($rgb >> 16) & 0xFF;
         $g = ($rgb >> 8) & 0xFF;
         $b = $rgb & 0xFF;
      
         $getArrayHSB = RGBtoHSB($r,$g,$b);
           
         $saturation = 9; //[0,10]
         $getArrayHSB[1] = $saturation*$getArrayHSB[1];
         $getArrayHSB[1] = max(0.0,min($getArrayHSB[1],1.0)); 
        
         $getArrayRGB = HSBtoRGB($getArrayHSB[0],
            $getArrayHSB[1],$getArrayHSB[2]);
      
         $hsbrgb = imagecolorallocate($im, 
            $getArrayRGB[0], $getArrayRGB[1], 
            $getArrayRGB[2]);
         $result = (($rgb & 0xff000000)|($hsbrgb));
    
         imagesetpixel($im, $i, $j, $result);    
      }
   }   
   header("Content-type: image/jpeg");
   imagejpeg($im);  
   ?>  
   

   <?php
 //Hue Filter
  $im = imagecreatefromjpeg("tree.jpg");
   $size = getimagesize("tree.jpg");
   $L=$size[0];
   $H=$size[1]; 
   
   for($j=0;$j<$H;$j++){
      for($i=0;$i<$L;$i++){  
         $rgb = imagecolorat($im, $i, $j); 
   
         $r = ($rgb >> 16) & 0xFF;
         $g = ($rgb >> 8) & 0xFF;
         $b = $rgb & 0xFF;
      
         $getArrayHSB = RGBtoHSB($r,$g,$b);
           
         $hue = 3; //[0,10]
         $getArrayHSB[0] = $hue*$getArrayHSB[0];
         $getArrayHSB[0] = max(0.0,min($getArrayHSB[0],360.0)); 
        
         $getArrayRGB = HSBtoRGB($getArrayHSB[0],
            $getArrayHSB[1],$getArrayHSB[2]);
      
         $hsbrgb = imagecolorallocate($im, 
            $getArrayRGB[0], $getArrayRGB[1], 
            $getArrayRGB[2]);
         $result = (($rgb & 0xff000000)|($hsbrgb));
    
         imagesetpixel($im, $i, $j, $result);         
      }
   }   
   header("Content-type: image/jpeg");
   imagejpeg($im);     
   ?>