trailingampit ensures a URL ends with a /amp/

gistfile1.txt

<?php
/**
 * Ensures the passed text, presumably an url, ends with '/amp/'
 */
if ( !function_exists( 'trailingampit' ) ) {
   function trailingampit( $string ) {
      return untrailingampit( $string ) . 'amp/';
   }
}

/**
 * Ensures the passed text, presumably an url, does not end with 'amp/' or 'amp'
 */
if ( !function_exists( 'untrailingampit' ) ) {
   function untrailingampit( $str ) {
      $result = preg_replace( '/(amp|amp\/)$/', '', $str );
      return $result;
   }
}