App setting php extension doesn't work


Hi, I need enable GMP extension for PHP in my website.

I already add this entry in the App Settings:
PHP_EXTENSIONS —> bin\php_gmp.dll
and uploaded such file in wwwroot\bin folder

Furthermore I have created a .user.ini file with this content:
and uploaded to wwwroot

However my website is throwing the error:
Uncaught Error: Call to undefined function gmp_init() in ...

How can Install/enable GMP extension correctly?