A source control system keeps track of changes to your files. It lets you review the history of how your code has changed, see who made what changes, and compare versions. With a source control system, two developers can work independently on changes and then combine them with ease.
A source control system is essential when more than one person is working on a project, but it is also useful for projects that you’re working on alone. Being able to “go back in time” and see what your code contained at a previous point is a lifesaver when you’re trying to figure out when a bug was introduced.
There are many popular source control systems, and which one you use will either be a matter of personal preference (for your own projects) or a foregone conclusion (when you’re working on an existing project that already has one). The code for the PHP engine itself is managed using the Git source control system. You can browse the PHP engine’s source code at http://git.php.net. Other popular source control systems include Mercurial and Subversion.
Source control systems excel at handling text files. Since your PHP code is essentially a collection of text files, there’s nothing special you need to do to make it play nicely with any popular source control system. Still, following some conventions will make your code easier to manage.
One convention is how to organize classes in files. If you’re writing object-oriented code, define only one class per file and make the filename the same as the class name (plus the .php extension). If you define classes inside namespaces, make a directory that corresponds to each namespace component, and arrange your files under those directories.
For example, a class named CheeseGrater goes in the file CheeseGrater.php. If you’ve defined that class in a Utensils namespace, then CheeseGrater.php goes in a Utensils subdirectory. Multiple levels of namespace mean multiple subdirectories. A class whose fully qualified name is \Kitchen\Utensils\CheeseGrater goes in the path Ki tchen/Utensils/CheeseGrater.php.
This convention is known as PSR-4. PSR means PHP Standard Recommendation. The PHP Standard Recommendations are conventions on coding style and organization that most major PHP projects use.
Source: Sklar David (2016), Learning PHP: A Gentle Introduction to the Web’s Most Popular Language, O’Reilly Media; 1st edition.