The generated PHP parser contains several instances of mb_* (multibyte string) functions:

     38 mb_substr(
      6 mb_strlen(
      3 mb_regex_encoding(
      1 mb_eregi(
      1 mb_ereg(
      1 mb_convert_encoding(

However, PHP is not always installed with the mbstring extension enabled, so these functions do not always exist.

WP core has polyfills that attempt to emulate mb_substr and mb_strlen if they are not present; the rest of these functions are newly introduced and will cause errors under PHP configurations without these functions enabled.

There are a few options for how to proceed:

Polyfill the new functions

Converting encodings and matching regexes both seem quite complicated.

Decide that this is no longer an issue

This code in core has a lot of history, and I am not sure how common it is to have PHP installed without the mbstring extension. (It's not enabled in a default build of PHP, but this hasn't been a problem in distributions of PHP that I've used).

In particular there is a mention of yoast.com having this configuration, but from 6 years ago.

Replace the new functions with something else

Fortunately, mb_convert_encoding is only used to convert a character code to a UTF-8 character, and [I am pretty sure that] the mb_ereg and mb_eregi calls are only used to match very simple regular expressions of the form ^[charlist] against single characters. So the simplest path forward is probably to remove these calls from phpegjs.