How to set environment variable before running script inside hooks/install?

I am create a simple charm to get my symfony2 webapp from git and deploy it on ec2.

After getting the source from git, I want to run composer to resolve dependencies but I keep getting this error:

INFO juju context.go:221 worker/uniter: HOOK   The HOME or COMPOSER_HOME environment variable must be set for composer to run correctly

I am running composer using below command inside my hooks/install script

juju-log "Running composer"
/usr/bin/php composer.phar install

I also tried

juju-log "Running composer"
COMPOSER_HOME=${app_dir};/bin/bash -c "/usr/bin/php composer.phar install"

How can I set HOME or COMPOSER_HOME so that this command can be executed?

HOME isn't set in the Juju hooks, only a few environment variables are.

You should be able to just run

COMPOSER_HOME="/path/you/want/to/be/home" php composer.phar install

which will set the environment variable before executing php.