the "numeric string" format function in ICU is here
http://icu-project.org/apiref/icu4c/classicu_1_1NumberFormat.html#aee2b88a01333eb699cbdca327e62841c

and of course the same goes for 
public string formatCurrency ( float $value , string $currency )
which should too accept "numeric strings"...

ICU has unum_formatDecimal()[1]:

| The syntax of the input number is a "numeric string" as defined
| in the Decimal Arithmetic Specification, available at
| http://speleotrove.com/decimal

From a quick glimpse, this looks quite different to PHP's numeric
strings, so supporting it wouldn't be straight forward.

[1] <https://unicode-org.github.io/icu-docs/apidoc/released/icu4c/unum_8h.html#a59870a322f012dc1b9d99cf8a7b708f1>

libicu has the code to create the Decimal Number for you.

You'd use https://unicode-org.github.io/icu-docs/apidoc/released/icu4c/classicu_1_1Formattable.html#a0505d26515fa1bbac3dd46a7e15e120e to create a Formattable from the string input, and then call formatUFormattable() on that: https://unicode-org.github.io/icu-docs/apidoc/released/icu4c/unum_8h.html#aec903c32ade7d632fb7125a5dfd667ea

Should be pretty straight-forward.

Hm, I don’t see so many differences between General Decimal Arithmetic numeric strings [1] and PHP numeric strings [2]:

1. PHP allows leading and trailing whitespace, GDA does not.
2. GDA includes special values (s)NaN and Inf(inity), PHP does not.

The syntax for finite numbers themselves is identical, as far as I can tell. (For instance, both allow lower- and uppercase E in exponential notation, empty digits before xor after decimal separator, only ASCII digits 0-9, leading zeroes anywhere…) Maybe this was different before PHP 8’s improved numeric strings [3].

[1]: http://speleotrove.com/decimal/daconvs.html#refnumsyn
[2]: https://www.php.net/manual/en/language.types.numeric-strings.php
[3]: https://wiki.php.net/rfc/saner-numeric-strings

This is the root cause of Wikimedia bug T268456, fwiw: https://phabricator.wikimedia.org/T268456

I have a patch written, I'll post it shortly.

There's a similar issue with currency formatting, but that requires an upstream patch to libicu: https://unicode-org.atlassian.net/browse/ICU-21479

> I have a patch written, I'll post it shortly.

Any update?  Also, better submit a pull request[1] instead of
attaching a patch for better visibility.

[1] <https://github.com/php/php-src/pulls>

github pr is now at https://github.com/php/php-src/pull/7782

Sorry for the long delay!  It got lost in my tree for a while.

I have a followup for formatCurrency at https://github.com/cscott/php-src/tree/bug76093-followup -- as currently written it required a new method in ICU, but as documented in https://unicode-org.atlassian.net/browse/ICU-21479 this can actually be done with the current ICU API in a certain way; I need to update the follow up to do things That Way.

I've added the followup which handles formatCurrency() to the same PR.  So both patches are there now.