Programmatically upload file to WordPress

upload.php

// downloads a file from a URL and saves it to the /tmp folder in the OS
$tmp_file = download_url( $url );

// the file info
$file = array(
  'name'     => $filename_with_extension,
  'type'     => 'image/png',
  'tmp_name' => $tmp_file, //src file
  'error'    => 0,
  'size'     => filesize( $tmp_file ),
);

// get the ID of the uploaded file
$id = wp_handle_sideload( $file, 0); // the file info, and the post to attach it to
if ( is_wp_error($id) ) {
  return false;
}

wp_update_attachment_metadata($id);