# Howto build a PHP builtin extension without having to rebuild full PHP distribution

Sometime, we need to dig into PHP extension code to make some adjustements. For
instance that is currently our case with the PHP openssl extension as provided
by older PHP versions (5.3, 5.4, 5.5) which is not compatible with openssl > 1.1...
We need patch the code to add openssl 1.1 support...

To do so, it would be tedious for us to have to recompile full PHP distribution
each time we want try a build of the openssl extension with our changes. A build
of the openssl extension take ~3 seconds while a build of the full PHP distribution
is far far longer...

So how to process exactly? Easy...

## Preparing PHP build environment

First, we need prepare our environment by installing the PHP build environment. This
should be simple as installing the PHP development package: apt-get phpX.Y-dev but...
there is always a but... the `phpX.Y-dev` package for our older PHP version (here 5.3)
depends on the `libssl1.0-dev` package while for the purpose of our backporting session,
we need the openssl 1.1 development libraries.

If we try to install the `phpX.Y-dev` package, this will also install the `libssl1.0-dev`
package on which it depend and if we try to install the `libssl-dev` package, this will
of course lead to it removal... Ouch... So what now? Should we rebuild the `phpX.Y-dev`
package to change dependencies for allowing installation of the `libssl-dev` package for
our backporting session? Of course yes, but a rebuild of the `phpX.Y-dev` in standard way
(think of dpkg-buildpackage invocation) would involve a build of a full PHP distribution
for various PHP SAPIs and we want avoid that (this would be too much time consuming).

Another solution, the one we choosen, is to unpack the `phpX.Y-dev` package manually to
edit the `DEBIAN/control` file and then repack it once done. This can be done quickly as
follows:
```
root@jenkins:/usr/local/src/SCM/# mkdir tmp
root@jenkins:/usr/local/src/SCM/# cp php5.3-dev_5.3.29-1~1.gbpbf5ab3_amd64 php5.3-dev_5.3.29-1~1.gbpbf5ab3_amd64.deb /tmp
root@jenkins:/usr/local/src/SCM/# cd tmp
root@jenkins:/usr/local/src/SCM/tmp# dpkg-deb -R php5.3-dev_5.3.29-1~1.gbpbf5ab3_amd64.deb php5.3-dev_5.3.29-1~1.gbpbf5ab3_amd64
root@jenkins:/usr/local/src/SCM/tmp# vi php5.3-dev_5.3.29-1~1.gbpbf5ab3_amd64/DEBIAN/control
root@jenkins:/usr/local/src/SCM/tmp# dpkg-deb -b php5.3-dev_5.3.29-1~1.gbpbf5ab3_amd64 php5.3-dev_5.3.29-1~1.gbpbf5ab3_amd64.deb
dpkg-deb: building package 'php5.3-dev' in 'php5.3-dev_5.3.29-1~1.gbpbf5ab3_amd64.deb'.
```
Then we can install the `libssl-dev` package and our new `phpX.Y-dev` package:
```
root@jenkins:/usr/local/src/SCM/tmp# apt-get install libssl-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
Recommended packages:
  libssl-doc
The following NEW packages will be installed:
  libssl-dev
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/1575 kB of archives.
After this operation, 7051 kB of additional disk space will be used.
Selecting previously unselected package libssl-dev:amd64.
(Reading database ... 36677 files and directories currently installed.)
Preparing to unpack .../libssl-dev_1.1.0f-3+deb9u2_amd64.deb ...
Unpacking libssl-dev:amd64 (1.1.0f-3+deb9u2) ...
Setting up libssl-dev:amd64 (1.1.0f-3+deb9u2) ...
root@jenkins:/usr/local/src/SCM/tmp# dpkg -i php5.3-dev_5.3.29-1~1.gbpbf5ab3_amd64.deb
Selecting previously unselected package php5.3-dev.
(Reading database ... 36763 files and directories currently installed.)
Preparing to unpack php5.3-dev_5.3.29-1~1.gbpbf5ab3_amd64.deb ...
Unpacking php5.3-dev (5.3.29-1~1.gbpbf5ab3) ...
Setting up php5.3-dev (5.3.29-1~1.gbpbf5ab3) ...
update-alternatives: using /usr/bin/php-config5.3 to provide /usr/bin/php-config (php-config) in auto mode
update-alternatives: using /usr/bin/phpize5.3 to provide /usr/bin/phpize (phpize) in auto mode
Processing triggers for man-db (2.7.6.1-2) ...
root@jenkins:/usr/local/src/SCM/tmp#
```

## Build of PHP openssl extension with our changes

To build the PHP openssl extension without having to build the full PHP distribution,
we need first prepare it for compilation as would will do with any external extension.
This is done by invoking the `phpize` script inside the extension source directory:
```
$ cd /usr/local/src/SCM/php/ext/openssl
$ mv config0.m4 config.m4
$ phpize
```
That's all what we have to do. We can now build the extension as usual:
```
$ make -j4
```

Happy coding ;)