Deploying a PHP application primarily implies deploying PHP code. Since PHP is an interpreted and not compiled language, the PHP applications deploy their code as-is, in source files. This means that there is no real build process involved when deploying an application, which further means that application deployment can be as easy as doing a git pullwithin a server web directory. Surely, things are never that simple, as we often have various other bits that need to fit in place when the code is deployed, such as databases, mounted drives, shared files, permissions, other services connected to our server, and so on.
We can easily imagine the complexity of having to manually deploy code from a single git repository onto dozens of web servers behind some load balancer at the same time. Such manual deployments will surely have negative implications, as we end up with a time in-between overall deployments, where one server might have newer versions of an application...