function make_thumb($src,$dest,$desired_width)
{

	/* read the source image */
	$source_image = imagecreatefromjpeg($src);
	$width = imagesx($source_image);
	$height = imagesy($source_image);
	
	/* find the "desired height" of this thumbnail, relative to the desired width  */
	$desired_height = floor($height*($desired_width/$width));
	
	/* create a new, "virtual" image */
	$virtual_image = imagecreatetruecolor($desired_width,$desired_height);
	
	/* copy source image at a resized size */
	imagecopyresized($virtual_image,$source_image,0,0,0,0,$desired_width,$desired_height,$width,$height);
	
	/* create the physical thumbnail image to its destination */
	imagejpeg($virtual_image,$dest, 83);
}

http://davidwalsh.name/create-image-thumbnail-php

it needs the GD2 library

Create a thumbnail function

php, image, function

PHP