php - What Are the Differences Between PSR-0 and PSR-4?

Question

Recently I've read about namespaces and how they are beneficial. I'm currently creating a project in Laravel and trying to move from class map autoloading to namespacing. However, I can't seem to grasp what the actual difference is between PSR-0 and PSR-4.

Some resources that I've read are...

• Battle of the Autoloaders
• Laracasts PSR-4 autoloading
• PSR-0
• PSR-4

What I understand:

• PSR-4 does not convert underscores to directory separators
• Certain specific rules of composer cause the directory structure to become complex which in turn makes PSR-0 namespacing verbose and thus PSR-4 was created

Examples explaining the difference would be appreciated.

Answer 1

They are very similar so it is not surprising that it's a bit confusing. The summary is that PSR-0 had some backwards compatibility features for PEAR-style classnames that PSR-4 dropped, as such it only supports namespaced code. On top of that PSR-4 does not force you to have the whole namespace as a directory structure, but only the part following the anchor point.

For example if you define that the AcmeFoo namespace is anchored in src/, with PSR-0 it means it will look for AcmeFooBar in src/Acme/Foo/Bar.php while in PSR-4 it will look for it in src/Bar.php, allowing for shorter directory structures. On the other hand some prefer to have the full directory structure to clearly see what is in which namespace, so you can also say that AcmeFoo is in src/Acme/Foo with PSR-4 which will gives you the equivalent of the PSR-0 behavior described above.

Long story short for new projects and for most intents and purposes, you can use PSR-4 and forget all about PSR-0.