PHP Startup error

I get randomly this error:
PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20100525/rar.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20100525/rar.so: cannot open shared object file: No such file or directory
The problem is in the Hosting service server or in the CMS?

Thank you

That's definitely a server configuration issue. rar.so is loading rar extracting support in to PHP. This is not something needed or used by Grav.

The hosting service provider deny any server error. Their solution is to change the CMS because is unsupported and there isn't any issue wth their server using Joomla or Wordpress... How this can be possible?

Can the composer.phar be involved? I've found RarArchive references into it

<?php
system('php bin/composer.phar --working-dir="path" --no-interaction --no-dev --prefer-dist -o install');

?>

With "path" the grav install dir triggers the error

If composer.phar is using RAR internally, that still means your hosting provider has it defined in the system's php.ini which is advertising that its available to use, when in fact its missing.

The fact that it runs joomla/wordpress without issue, only means that those are not making use of any rar-based command.

The error clearly states it's a missing file issue, and that can only come from the php.ini, which means its a server issue.

Basically it's still a misconfiguration of your hosting provider, and if they are saying otherwise, they don't know what they are talking about and you really should look for a more knowledgable provider.

As suggested by Gert, you can also try to run php -v from the shell prompt and see if that reports any errors