The imagefilter() function can be used to apply various fun and/or useful effects to an existing image or photograph. The selection of effects is:

Parameter	Description	Arguments
IMG_FILTER_NEGATE	Reverses All Colors of the Image
IMG_FILTER_GRAYSCALE	Converts the Image Into Grayscale
IMG_FILTER_BRIGHTNESS	Changes the Brightness of the Image	Use arg1 to Set the Level of Brightness
IMG_FILTER_CONTRAST	Changes the Contrast of the Image	Use arg1 to Set the Level of Contrast
IMG_FILTER_COLORIZE	Like IMG_FILTER_GRAYSCALE, Except You Can Specify the Color	Use arg1, arg2 & arg3 in the form of red, blue, green and arg4 for the alpha channel. The range for each color is 0 to 255.
IMG_FILTER_EDGEDETECT	Uses Edge Detection to Highlight the Edges of the Image
IMG_FILTER_EMBOSS	Embosses the Image
IMG_FILTER_GAUSSIAN_BLUR	Blurs the Image Using the Gaussian Method
IMG_FILTER_SELECTIVE_BLUR	Blurs the Image
IMG_FILTER_MEAN_REMOVAL	Uses Mean Removal to Achieve a "Sketchy" Effect.
IMG_FILTER_SMOOTH	Makes the Image Smoother	Use arg1 to Set the Level of Smoothness
IMG_FILTER_PIXELATE	Applies Pixelation Effect to Image	Use arg1 to Set Block Size & arg2 to Set Pixelation Effect Mode

The syntax is: imagefilter (resource $image, int $filtertype [, int $arg1 [, int $arg2 [, int $arg3 [, int $arg4 ]]]])

Let's look at the code:

<?php
  header('Content-type: image/jpeg');
  $jpg_image = imagecreatefromjpeg('sky.jpg');
  imagefilter($jpg_image, IMG_FILTER_GRAYSCALE);
  imagejpeg($jpg_image);
  imagedestroy($jpg_image);
?>

And now you can see the original image and the programmatically modified grayscale image.

A "sketch" effect can be applied. Example: imagefilter($jpg_image, IMG_FILTER_MEAN_REMOVAL);

Brightness can be heightened. Example: imagefilter($jpg_image, IMG_FILTER_BRIGHTNESS, 50);

Colors can be modified. Example: imagefilter($jpg_image, IMG_FILTER_COLORIZE, 0, 50, 0);

And more!

Summary:

Function	Description
imagefilter()	Applies a Filter to An Image