PHP 7.3 add support for Symfony 4

[titomiguelcosta]

Unfortunately the extension simplexml is not available on version PHP 7.3, so I can not use the layer to run a Symfony 4 application. There are a few extensions that are required by Symfony 4. Could we support it, like you do for PHP 7.1?

So that you know, I was able to run it using PHP 7.1 by having a php.ini at the root of the project that enabled the needed extensions and after changing the cache and log dirs to use /tmp.

[txase]

@titomiguelcosta Would you be able to investigate what it would take to install simplexml (and any other required extensions for Symfony?) in the build script for PHP 7.3? That's a piece of data we're missing to be able to drive this forward.

Thanks!

[titomiguelcosta]

@txase I submitted the PR #39 that seems to fix the issue, let me know what you think