PHP 8.2: ${var} string interpolation deprecated

[tenzap]

Ref: https://php.watch/versions/8.2/$%7Bvar%7D-string-interpolation-deprecated

This is used in system/libraries/Xmlrpcs.php

	/**
	 * Multi-call Function: Error Handling
	 *
	 * @param	mixed
	 * @return	object
	 */
	public function multicall_error($err)
	{
		$str = is_string($err) ? $this->xmlrpcstr["multicall_${err}"] : $err->faultString();
		$code = is_string($err) ? $this->xmlrpcerr["multicall_${err}"] : $err->faultCode();

		$struct['faultCode'] = new XML_RPC_Values($code, 'int');
		$struct['faultString'] = new XML_RPC_Values($str, 'string');

		return new XML_RPC_Values($struct, 'struct');
	}

Please fix it to prepare support for php 8.2

[kenjis]

Why don't you send a PR?