Warning: Use of undefined constant INTL_IDNA_VARIANT_UTS46

```
( ! ) Warning: Use of undefined constant INTL_IDNA_VARIANT_UTS46 - assumed
'INTL_IDNA_VARIANT_UTS46' (this will throw an Error in a future version of PHP) in
/home/public_html/wp-content/plugins/amp/includes/class-amp-http.php on line 205
```

From looking online, this may be due to a really old version of ICU running on the machine, but something that we may want to account for?
