Temporary failure in name resolution : Host name lookup failure
Solution 1:
PHP is having trouble accessing either /etc/hosts or /etc/resolv.conf: there's a long standing issue in PHP related to this specific error. The fix is to try restarting Apache or whatever is invoking PHP, or to make sure /etc/hosts and /etc/resolv.conf are readable by what's invoking PHP.
Solution 2:
I just experienced the same error and
service httpd restart
did the trick...
Solution 3:
% dig @208.67.222.222 smtp.gmail.com +short
gmail-smtp-msa.l.google.com.
209.85.201.109
209.85.201.111
%
Now, try using Xdebug to see where the problem is exactly....
<?php
xdebug_start_trace('/tmp/lookup-trace.log');
$ip = gethostbyname('smtp.gmail.com');
xdebug_stop_trace();
die($IP);
?>
Anything good in the logs?