PSR-4 Meta Document

PSR-4 Meta Document

1. Summary

The purpose is to specify the rules for an interoperable PHP autoloader that
maps namespaces to file system paths, and that can co-exist with any other SPL
registered autoloader. This would be an addition to, not a replacement for,

2. Why Bother?

History of PSR-0

The PSR-0 class naming and autoloading standard rose out of the broad
acceptance of the Horde/PEAR convention under the constraints of PHP 5.2 and
previous. With that convention, the tendency was to put all PHP source classes
in a single main directory, using underscores in the class name to indicate
pseudo-namespaces, like so:

            Baz.php     # VendorFoo_Bar_Baz
            Gir.php     # Vendor_Dib_Zim_Gir

With the release of PHP 5.3 and the availability of namespaces proper, PSR-0
was introduced to allow both the old Horde/PEAR underscore mode and the use
of the new namespace notation. Underscores were still allowed in the class
name to ease the transition from the older namespace naming to the newer naming,
and thereby to encourage wider adoption.

            Baz.php     # VendorFoo_Bar_Baz
            Gir.php     # VendorDib_Zim_Gir
            V2.php      # Irk_Operation\Impending_Doom\V2

This structure is informed very much by the fact that the PEAR installer moved
source files from PEAR packages into a single central directory.

Along Comes Composer

With Composer, package sources are no longer copied to a single global
location. They are used from their installed location and are not moved
around. This means that with Composer there is no “single main directory” for
PHP sources as with PEAR. Instead, there are multiple directories; each
package is in a separate directory for each project.

To meet the requirements of PSR-0, this leads to Composer packages looking
like this:

                        ClassName.php       # Vendor_Name\Package_Name\ClassName
                        ClassNameTest.php   # Vendor_Name\Package_Name\ClassNameTest

The “src” and “tests” directories have to include vendor and package directory
names. This is an artifact of PSR-0 compliance.

Many find this structure to be deeper and more repetitive than necessary. This
proposal suggests that an additional or superseding PSR would be useful so
that we can have packages that look more like the following:

                ClassName.php       # Vendor_Name\Package_Name\ClassName
                ClassNameTest.php   # Vendor_Name\Package_Name\ClassNameTest

This would require an implementation of what was initially called
“package-oriented autoloading” (as vs the traditional “direct class-to-file

Package-Oriented Autoloading

It’s difficult to implement package-oriented autoloading via an extension or
amendment to PSR-0, because PSR-0 does not allow for an intercessory path
between any portions of the class name. This means the implementation of a
package-oriented autoloader would be more complicated than PSR-0. However, it
would allow for cleaner packages.

Initially, the following rules were suggested:

  1. Implementors MUST use at least two namespace levels: a vendor name, and
    package name within that vendor. (This top-level two-name combination is
    hereinafter referred to as the vendor-package name or the vendor-package

  2. Implementors MUST allow a path infix between the vendor-package namespace
    and the remainder of the fully qualified class name.

  3. The vendor-package namespace MAY map to any directory. The remaining
    portion of the fully-qualified class name MUST map the namespace names to
    identically-named directories, and MUST map the class name to an
    identically-named file ending in .php.

Note that this means the end of underscore-as-directory-separator in the class
name. One might think underscores should be honored as they are under
PSR-0, but seeing as their presence in that document is in reference to
transitioning away from PHP 5.2 and previous pseudo-namespacing, it is
acceptable to remove them here as well.

3. Scope

3.1 Goals

  • Retain the PSR-0 rule that implementors MUST use at least two namespace
    levels: a vendor name, and package name within that vendor.

  • Allow a path infix between the vendor-package namespace and the remainder of
    the fully qualified class name.

  • Allow the vendor-package namespace MAY map to any directory, perhaps
    multiple directories.

  • End the honoring of underscores in class names as directory separators

3.2 Non-Goals

  • Provide a general transformation algorithm for non-class resources

4. Approaches

4.1 Chosen Approach

This approach retains key characteristics of PSR-0 while eliminating the
deeper directory structures it requires. In addition, it specifies certain
additional rules that make implementations explicitly more interoperable.

Although not related to directory mapping, the final draft also specifies how
autoloaders should handle errors. Specifically, it forbids throwing exceptions
or raising errors. The reason is two-fold.

  1. Autoloaders in PHP are explicitly designed to be stackable so that if one
    autoloader cannot load a class another has a chance to do so. Having an autoloader
    trigger a breaking error condition violates that compatibility.

  2. class_exists() and interface_exists() allow “not found, even after trying to
    autoload” as a legitimate, normal use case. An autoloader that throws exceptions
    renders class_exists() unusable, which is entirely unacceptable from an interoperability
    standpoint. Autoloaders that wish to provide additional debugging information
    in a class-not-found case should do so via logging instead, either to a PSR-3
    compatible logger or otherwise.


  • Shallower directory structures

  • More flexible file locations

  • Stops underscore in class name from being honored as directory separator

  • Makes implementations more explicitly interoperable


  • It is no longer possible, as under PSR-0, to merely examine a class name to
    determine where it is in the file system (the “class-to-file” convention
    inherited from Horde/PEAR).

4.2 Alternative: Stay With PSR-0 Only

Staying with PSR-0 only, although reasonable, does leave us with relatively
deeper directory structures.


  • No need to change anyone’s habits or implementations


  • Leaves us with deeper directory structures

  • Leaves us with underscores in the class name being honored as directory

4.3 Alternative: Split Up Autoloading And Transformation

Beau Simensen and others suggested that the transformation algorithm might be
split out from the autoloading proposal so that the transformation rules
could be referenced by other proposals. After doing the work to separate them,
followed by a poll and some discussion, the combined version (i.e.,
transformation rules embedded in the autoloader proposal) was revealed as the


  • Transformation rules could be referenced separately by other proposals


  • Not in line with the wishes of poll respondents and some collaborators

4.4 Alternative: Use More Imperative And Narrative Language

After the second vote was pulled by a Sponsor after hearing from multiple +1
voters that they supported the idea but did not agree with (or understand) the
wording of the proposal, there was a period during which the voted-on proposal
was expanded with greater narrative and somewhat more imperative language. This
approach was decried by a vocal minority of participants. After some time, Beau
Simensen started an experimental revision with an eye to PSR-0; the Editor and
Sponsors favored this more terse approach and shepherded the version now under
consideration, written by Paul M. Jones and contributed to by many.

Compatibility Note with PHP 5.3.2 and below

PHP versions before 5.3.3 do not strip the leading namespace separator, so
the responsibility to look out for this falls on the implementation. Failing
to strip the leading namespace separator could lead to unexpected behavior.

5. People

5.1 Editor

  • Paul M. Jones, Solar/Aura

5.2 Sponsors

  • Phil Sturgeon, PyroCMS (Coordinator)
  • Larry Garfield, Drupal

5.3 Contributors

  • Andreas Hennings
  • Bernhard Schussek
  • Beau Simensen
  • Donald Gilbert
  • Mike van Riel
  • Paul Dragoonis
  • Too many others to name and count

6. Votes