PSR-0: Autoloading Standard

Autoloading Standard

Deprecated - As of 2014-10-21 PSR-0 has been marked as deprecated. PSR-4 is now recommended as an alternative.

The following describes the mandatory requirements that must be adhered to for autoloader interoperability.


  • A fully-qualified namespace and class must have the following structure \<Vendor Name>\(<Namespace>\)*<Class Name>
  • Each namespace must have a top-level namespace ("Vendor Name").
  • Each namespace can have as many sub-namespaces as it wishes.
  • Each namespace separator is converted to a DIRECTORY_SEPARATOR when loading from the file system.
  • Each _ character in the CLASS NAME is converted to a DIRECTORY_SEPARATOR. The _ character has no special meaning in the namespace.
  • The fully-qualified namespace and class is suffixed with .php when loading from the file system.
  • Alphabetic characters in vendor names, namespaces, and class names may be of any combination of lower case and upper case.


  • \Doctrine\Common\IsolatedClassLoader => /path/to/project/lib/vendor/Doctrine/Common/IsolatedClassLoader.php
  • \Symfony\Core\Request => /path/to/project/lib/vendor/Symfony/Core/Request.php
  • \Zend\Acl => /path/to/project/lib/vendor/Zend/Acl.php
  • \Zend\Mail\Message => /path/to/project/lib/vendor/Zend/Mail/Message.php

Underscores in Namespaces and Class Names

  • \namespace\package\Class_Name => /path/to/project/lib/vendor/namespace/package/Class/Name.php
  • \namespace\package_name\Class_Name => /path/to/project/lib/vendor/namespace/package_name/Class/Name.php

The standards we set here should be the lowest common denominator for painless autoloader interoperability. You can test that you are following these standards by utilizing this sample SplClassLoader implementation which is able to load PHP 5.3 classes.

Example Implementation

Below is an example function to simply demonstrate how the above proposed standards are autoloaded.


function autoload($className)
    $className = ltrim($className, '\\');
    $fileName  = '';
    $namespace = '';
    if ($lastNsPos = strrpos($className, '\\')) {
        $namespace = substr($className, 0, $lastNsPos);
        $className = substr($className, $lastNsPos + 1);
        $fileName  = str_replace('\\', DIRECTORY_SEPARATOR, $namespace) . DIRECTORY_SEPARATOR;
    $fileName .= str_replace('_', DIRECTORY_SEPARATOR, $className) . '.php';

    require $fileName;

SplClassLoader Implementation

The following gist is a sample SplClassLoader implementation that can load your classes if you follow the autoloader interoperability standards proposed above. It is the current recommended way to load PHP 5.3 classes that follow these standards.