PHP Namespaces & Standard PHP Library
Namespaces
The PHP community has a lot of developers creating lots of code. This means that one library’s PHP code might use the same class name as another. When both libraries are used in the same namespace, they collide and cause trouble.
Namespaces solve this problem. As described in the PHP reference manual, namespaces may be compared to operating system directories that namespace files; two files with the same name may co-exist in separate directories. Likewise, two PHP classes with the same name may co-exist in separate PHP namespaces. It’s as simple as that.
It is important for you to namespace your code so that it may be used by other developers without fear of colliding with other libraries.
One recommended way to use namespaces is outlined in PSR-4, which aims to provide a standard file, class and namespace convention to allow plug-and-play code.
In October 2014 the PHP-FIG deprecated the previous autoloading standard: PSR-0. Both PSR-0 and PSR-4 are still perfectly usable. The latter requires PHP 5.3, so many PHP 5.2-only projects implement PSR-0.
If you’re going to use an autoloader standard for a new application or package, look into PSR-4.
Standard PHP Library
The Standard PHP Library (SPL) is packaged with PHP and provides a collection of classes and interfaces. It is made up primarily of commonly needed datastructure classes (stack, queue, heap, and so on), and iterators which can traverse over these datastructures or your own classes which implement SPL interfaces.