Client-side code runs on a client’s computer whereas server-side code runs on a web server before a webpage is sent to the client. A user first requests a webpage from a server, the server may then run some operations or processes, then afterward the webpage is sent to the user.
PHP is a well-known server-side programming language, which means it is executed on the server-side prior to the user receiving the webpage. Whatever code is written in PHP will not be visible to the user, on the client-side, as the code will have already been executed and removed before the webpage is sent to the user. This can allow for more private code, such as encryption algorithms, to be written in PHP and not be accessed, read, or reverse-engineered by the user.