A function that takes a vimeo url and returns the video ID

get_vimeo_id.php

<?php
function get_vimeo_id( $url ) {
  /**
   * Matches:
   * https://player.vimeo.com/video/123456789
   * https://vimeo.com/123456789
   * https://www.vimeo.com/123456789
   */
  preg_match( '/(vimeo\.com\/)(?:video\/)?([\d]+)/', $url, $matches );
  $vimeo_id = $matches[2];

  return $vimeo_id;
}