Sometimes, simply running php composer install
or php composer update
will throw an error at building bootstrap.php time. To resolve this issue, proceed like this:
First, run:
php composer update --no-scripts
This will install all the vendors without launching any script that could create errors Then simply do:
php vendor/sensio/distribution-bundle/Sensio/Bundle/DistributionBundle/Resources/bin/build_bootstrap.php
This will build the bootstrap file needed for Symfony2 to run properly.