The following code ran fine in PHP under apache but is failing to run under nginx/php-fpm:

exec( "/usr/bin/xvfb-run -a -s '-screen 0 640x480x16' /usr/bin/wkhtmltopdf --page-size Letter --margin-top 10 --margin-bottom 10 $url $tempPDF" );

I've been through all the configuration files but nothing I've done seems to work, not even the error logs are reporting anything.

This server was initially configured by the iRedMail installation script.

I would also like to add that this works fine from the command line.

This issue was solved by adding the following line to /etc/php-fpm.d/www.conf:

env[PATH] = '/usr/local/bin:/usr/bin:/bin'