Simple plural function php

make_plural.php

<?php
function plural( $string ) {
  
  $last_letter = substr($string, -1); // Get the last letter of the taxonomy name
  if ( $last_letter != 's' ) :
    // If it ends in a "Y" replace that with "IES" otherwise add an "S"
    $string = ( $last_letter == 'y' ) ? substr($string, 0, -1) . 'ies' : $string . 's';
  endif;
  
  return $string;
}