Background

As Matti recently wrote about, Gentoo will start supporting multiple concurrent PHP installs through minor version slotting.

PHP allows for ABI breaks for every minor version release. This means that for every upgrade to a new minor version, e.g from 5.3 to 5.4, you have to recompile your PECL packages and other extensions such as suhosin, xdebug and eaccellerator. Many extensions, especially suhosin, xdebug and opcode caches, often need to be rewritten in order to work properly with the new ABIs.

In addition, PHP tend to ship with quite a lot of new functionality with minor versions. PHP 5.3 saw the coming of closures, namespaces and late static bindings. In addition, quite a lot of functionality got deprecated and a few functions changed behaviour.

Because of the change in behaviour and the ABI changes breaking extensions, many developers, me included, want to be able to experiment with the new functionality of a new minor version, while still developing using the previous, more stable minor version.

Gentoo currently support major version slotting, but as outlined above, this is not really sufficient. The implementation of the major version slotting is based around having one category for every slot, e.g dev-php4 and dev-php5, and copying packages into the categories that each package supports. This means that you would have both a package called dev-php4/phpunit and dev-php5/phpunit.

In addition there is a dev-php category for packages that work across all PHP versions. However, all PEAR packages, regardless of whether it works across all PHP versions or not, is placed in dev-php, breaking this scheme.

Choosing which slot to use is done through php-select found in the app-admin/php-toolkit package. It allows you to chose which php installation to use for CLI, mod_php (apache2) etc. Many of you are probably not aware of this since it has been a while since php4 support was dropped.

Enter minor version slotting

Because of the ABI breaks and the change of behaviour in between minor versions of PHP, Gentoo will soon support minor version slotting. This will be implemented concurrently with cleaning up PHP related eclasses and merging the PHP related package categories into one single category.

The implementation of minor version slotting is founded on an eselect module for PHP.

Building extensions for multiple slots

After the install of each new PHP slot, the eselect module will enable the slot for building PECL packages. This allows users to chose which PHP installations extensions should be built for.

eselect php enable php5.3 enables extension-building for PHP 5.3, while eselect php disable php5.2 disables extension building for PHP 5.2.

PHP ebuilds will automatically enable extension-building for newly installed slots.

Compared to the current scheme of major version slotting, the one disadvantage of this approach is that you cannot build extensions with different USE flags for individual slots. All extensions have to be built with identical configurations.

NOTE: An alternative to this is to build all extensions for all installed slots. This is actually what happens if you do not disable slots manually.

This was changed to using USE_EXPAND instead. See: Building PHP extensions against multiple slots using PHP_TARGETS

Choosing which PHP installation to use

In order to use a new PHP installation for CLI or apache2 you will have to switch manually using, once again, eselect-php: eselect php set apache2

This might be seen as a bit of a configuration overhead, but with the current rate of PHP development, you shouldn’t have to run these commands more than once every other year.