If it is an undefined function then it means the function declaration cannot be found.
Which means your PHP version is missing MSSQL support (by default it's not usually included in most builds).
In windows it's as simple as uncommenting the library extension in "php.ini" and restarting apache.
Code:
extension=php_mssql.dll
In linux there should be a module using an RPM or YUM or it may need to be re-compiled with mssql support.