After building a project using Composer, you might ask yourself what the lowest supported PHP version amongst all the included libraries is. The highest number would also be the the minimum version PHP version that the project would run on.
To quickly check this in composer, run this from the project root:
composer show -i -t | grep -o "\-\-\(ext-\|php\).\+" | sort | uniq | cut -d- -f3-
Your output might look something like this:
php >=5.3.0 php >=5.3.9 php ^5.5 || ^7.0 php >=5.5.9
By checking the list of versions, we can see that 5.5.9 would be the lowest PHP version we would need to run this project.