Don't ask me why I need to run this php version (shame on me) but rather help me figure out if is a good/acceptable decision taking into consideration that this will be a large project and we might need to install many 3rd party packages.
Note: I know Laravel 5.5 is launching my concern is more with the PHP version with exactly Laravel 5.4
as Emil Moe mentioned.
php.net/supported-versions.php
basically you don't get any security patches by not switching to php7 and I personally would already switch to php7.1 if you look at the timeline.
Emil Moe
Senior Data Engineer
You should upgrade to php7, many extensions already require that now