A fresh linux OS running inside a vagrant box will not resolve domain names itself.
Using PHP, you'll get some "
PHP php_network_getaddresses: getaddrinfo failed: No such host is known</code>" errors.
To fix it, just change your /etc/network/interfaces conf:
auto eth0 iface eth0 inet dhcp dns-search google.com dns-nameservers dnsserverip
Then restart the network manager :
sudo service network restart
You'll be disconnected from your previous vagrant ssh session, but that's fine, just re-log, and that's it.