自动加载是一种机制，无需依赖手动编写PHP代码。参考?PHP手册自动加载，一旦自动加载器被定义，你试图使用一个没有定义的类或接口的情况下，它会自动被调用。
使用自动加载，在项目中你不必担心类的存放位置。定义一个良好定义的自动加载器，您不需要考虑一个类文件相对于当前类文件的位置，您只需使用类，自动加载器将自动查找文件。
此外，自动加载，确保只加载一次，提升了性能 -所以可以用它替代require_once()。
Zend Framework 鼓励使用自动加载，并提供了许多工具实现自动加载代码库以及应用程序代码。下面将介绍这些工具，以及如何有效地使用它们。
To understand autoloading in Zend Framework, first you need to understand the relationship between class names and class files.
Zend Framework has borrowed an idea from ? PEAR, whereby class names have a 1:1 relationship with the filesystem. Simply put, the underscore character ("_") is replaced by a directory separator in order to resolve the path to the file, and then the suffix ".php" is added. For example, the class "Foo_Bar_Baz" would correspond to "Foo/Bar/Baz.php" on the filesystem. The assumption is also that the classes may be resolved via PHP'sinclude_path setting, which allows both include() and require() to find the filename via a relative path lookup on the include_path.
Additionally, per PEAR as well as the ? PHP project, we use and recommend using a vendor or project prefix for your code. What this means is that all classes you write will share a common class prefix; for example, all code in Zend Framework has the prefix "Zend_". This naming convention helps prevent naming collisions. Within Zend Framework, we often refer to this as the "namespace" prefix; be careful not to confuse it with PHP's native namespace implementation.
Zend Framework follows these simple rules internally, and our coding standards encourage that you do so as well for all library code.
Zend Framework's autoloading support, provided primarily via Zend_Loader_Autoloader, has the following goals and design elements:
Provide namespace matching. If the class namespace prefix is not in a list of registered namespaces, return FALSE immediately. This allows for more optimistic matching, as well as fallback to other autoloaders.
Allow the autoloader to act as a fallback autoloader. In the case where a team may be widely distributed, or using an undetermined set of namespace prefixes, the autoloader should still be configurable such that it will attempt to match any namespace prefix. It will be noted, however, that this practice is not recommended, as it can lead to unnecessary lookups.
Allow toggling error suppression. We feel -- and the greater PHP community does as well -- that error suppression is a bad idea. It's expensive, and it masks very real application problems. So, by default, it should be off. However, if a developer insiststhat it be on, we allow toggling it on.
Allow specifying custom callbacks for autoloading. Some developers don't want to useZend_Loader::loadClass() for autoloading, but still want to make use of Zend Framework's mechanisms. Zend_Loader_Autoloader allows specyfing an alternate callback for autoloading.
Allow manipulation of the SPL autoload callback chain. The purpose of this is to allow specifying additional autoloaders -- for instance, resource loaders for classes that don't have a 1:1 mapping to the filesystem -- to be registered before or after the primary Zend Framework autoloader.