Composer installed, but get /usr/bin/env: php: No such file or directory

You need to add /usr/local/bin to your PATH variable. The easiest way is to throw it in your profile or bash_profile located at either:

• ~/.profile
• ~/.bash_profile

You would add the following to one of those files:

export PATH="$PATH:/usr/local/bin/"

For more details, see: https://unix.stackexchange.com/questions/26047/how-to-correctly-add-a-path-to-path

If you are logged in when you add it, you can force Linux to read the file again and update the path (once the changes are made) by using source from the bash prompt:

source ~/.bash_profile

As for the php7 vs. php issue, as Alex suggested, you can make a symlink so it works kinda like an alias.

This worked for me [Centos 7 with php 7.1] :
yum install php71w-cli

You need to install the cli package.

yum install php71u-cli is what I needed to do for IUS php.

As @alexhowansky suggested in a comment, I ran the following command:

sudo ln -s /usr/bin/php71 /usr/bin/php

Now the composer command works.