(PHP 4, PHP 5, PHP 7)
imagecopyresized — Copy and resize part of an image
$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
   )
   imagecopyresized() copies a rectangular
   portion of one image to another image.
   dst_image is the destination image,
   src_image is the source image identifier.
  
   In other words, imagecopyresized() will take a 
   rectangular area from src_image of width 
   src_w and height src_h at 
   position (src_x,src_y) 
   and place it in a rectangular area of dst_image 
   of width dst_w and height dst_h
   at position (dst_x,dst_y).
  
   If the source and destination coordinates and width and heights
   differ, appropriate stretching or shrinking of the image fragment
   will be performed. The coordinates refer to the upper left
   corner. This function can be used to copy regions within the
   same image (if dst_image is the same as
   src_image) but if the regions overlap the
   results will be unpredictable.
  
dst_imageDestination image link resource.
src_imageSource image link resource.
dst_xx-coordinate of destination point.
dst_yy-coordinate of destination point.
src_xx-coordinate of source point.
src_yy-coordinate of source point.
dst_wDestination width.
dst_hDestination height.
src_wSource width.
src_hSource height.
   Returns TRUE on success or FALSE on failure.
  
Example #1 Resizing an image
This example will display the image at half size.
<?php
// File and new size
$filename = 'test.jpg';
$percent = 0.5;
// Content type
header('Content-Type: image/jpeg');
// Get new sizes
list($width, $height) = getimagesize($filename);
$newwidth = $width * $percent;
$newheight = $height * $percent;
// Load
$thumb = imagecreatetruecolor($newwidth, $newheight);
$source = imagecreatefromjpeg($filename);
// Resize
imagecopyresized($thumb, $source, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
// Output
imagejpeg($thumb);
?>
The above example will output something similar to:
 
     The image will be output at half size, though better quality could be obtained using imagecopyresampled().
Note:
There is a problem due to palette image limitations (255+1 colors). Resampling or filtering an image commonly needs more colors than 255, a kind of approximation is used to calculate the new resampled pixel and its color. With a palette image we try to allocate a new color, if that failed, we choose the closest (in theory) computed color. This is not always the closest visual color. That may produce a weird result, like blank (or visually blank) images. To skip this problem, please use a truecolor image as a destination image, such as one created by imagecreatetruecolor().
imagecopyresampled() - Copy and resize part of an image with resampling