I wrote this function long time ago and I have used it so many since then. I decided to publish it.

You can find plenty of similar functions but this one is a little bit different. As a parameter you specify new MAX dimensions so image will be scaled based on its height or width. I believe I have enough comments in the code.

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public function resize($source_image, $target_image, $new_width, $new_height) {
  // get current size
  list($width, $height) = @getimagesize($source_image);
 
  if ($width > $new_width || $height > $new_height) {
    $scale1 = $width / $new_width;
    $scale2 = $height / $new_height;
 
    // check if we should scale using width or height
    $scale = $scale1 > $scale2 ? $scale1 : $scale2;
 
    // make sure we scale correctly
    $new_height = floor($height / $scale);
    $new_width = floor($width / $scale);
 
    // Load
    $thumb = imagecreatetruecolor($new_width, $new_height);
    $source = imagecreatefromjpeg($source_image);
 
    // Resize
    imagecopyresampled($thumb, $source, 0, 0, 0, 0, $new_width, $new_height, $width, $height);
 
    // Save the file at 95% quality
    imagejpeg($thumb, $target_image, 95);
  } else {
    // image small already - don't resize
    copy($source_image, $target_image);
  }
}