Not for 'how-to' coding questions but PHP theory instead, this forum is here for those of us who wish to learn about design aspects of programming with PHP.
Not sure what language you are talking about, so I will assume PHP. Functions that require external libraries need to have the library module loaded and initialized in the php.ini configuration file. If you look in the php.ini file (or the httpd.conf for that matter) you can see many additional modules are commented out and not loaded by default. They can also be compiled in, but usually they are loaded when PHP is loaded.