I'll be working soon on a project and I just wanted to ask this before. The project is on symfony 2.8 with php 5.4. The goal is to upgrade symfony to 3.4 (maybe 4.x later) and php to 7.x (probably 7.2). So here is my question : is there a better order to do this? Php 5.4 to 7.2 first or symfony 2.8 to 3.4 (or it doesn't matter)? Thanks for your advices.
First of all – you won't be able to run Symfony 3.4 project on PHP5.4.
As of 2018-11-26
The latest Symfony ^3.4 version is v3.4.19 which requires php: ^5.5.9|>=7.0.8.
On the other hand latest Symfony ^2.8 version is v2.8.48 which requires php: >=5.3.9.
Conclusion
You won't be able to run Symfony 3.4 project on PHP 5.4, but in theory you will be able to run Symfony 2.8 project with PHP 7.2.
You should also consider taking a look at: http://php.net/manual/en/migration70.incompatible.php