What are/How to use system calls?

Hello, new to php and one thing I wanted to do was have a page accessed and download a directory via zip automatically (Specify dir. ahead of time) well, no luck, but somebody mentioned system calls. As if my linux were to have zip/unzip capabilities on the server, I can use a 'system call' to get it to work? How would I do such a thing? Imagine, zip.php is accessed by a user. Immediately, it has a variable 'dir' which = a directory like /directory and it turns that dir. into a zip how would I call that utility to do it? As when I ask how to use php to do zips, the zend.com articles and what not either don't work or are too confusing system call sounds great. It would then work for pdf to html and endless possibilities. Correct? Or am I missing something? Thanks