Image thumbnails are basically a smaller size copy of a larger image. They can be created using PHP GD Library functions alongside a file system function that we already learned about.

The getimagesize() file function will return the dimensions of the image specified. It can be used together with the list() function to minimize code and effort.

The imagecopyresampled() function can be used to stick a smaller version of an existing image into a new (smaller) image, thus creating a thumbnail.

The syntax is: imagecopyresampled(resource $dst_image, resource $src_image, int $dst_x, int $dst_y, int $src_x, int $src_y, int $dst_w, int $dst_h, int $src_w, int $src_h)

dst_image = Destination Image
src_image = Source Image
dst_x = X Coordinate of Destination Image
dst_y = Y Coordinate of Destination Image
src_x = X Coordinate of Source Image
src_y = Y Coordinate of Source Image
dst_w = Destination Width
dst_h = Destination Height
src_w = Source Width
src_h = Source Height

Putting it together, the script will look like this:

<?php
  header('Content-type: image/jpeg');
  $file = 'sky.jpg';

  $new_width = 150;
  $new_height = 150;
  list($old_width, $old_height) = getimagesize($file);

  $new_image = imagecreatetruecolor($new_width, $new_height);
  $old_image = imagecreatefromjpeg($file);

  imagecopyresampled($new_image, $old_image, 0, 0, 0, 0, $new_width, $new_height, $old_width, $old_height);

  imagejpeg($new_image);
  imagedestroy($old_image);
  imagedestroy($new_image);
?>

And the result will look like this.

Summary:

Function	Description
imagecopyresampled()	Copy & Resize Part of An Image With Resampling