For a possible solutions see here.
Excerpt from post...
This is what I did to make unoconv work through apache / php code running on Cent OS 6.2 (unoconv version 0.6 and LibreOffice 3.4.5.2): (This is only a workaround - root cause is not known to me)
Change apache user from /sbin/nologin to /bin/bash (This is done in /etc/passwd file) Add a new user unoconv Added a new file /etc/sudoers.d/unoconv with the following contents:
apache ALL=(unoconv) NOPASSWD: /usr/bin/unoconv (note that my unoconv program is in this location /usr/bin/unoconv - you find it using which unoconv)
Using visudo comment out the followin line (by adding a # at the start of the line)
#Defaults requiretty
Restart sshd and httpd services
Run unoconv like this with php exec() function (you would need to change the input file name and output directory):
exec('sudo -u unoconv /usr/bin/unoconv -f pdf -o bankgenerated Teacher_bulk_upload.csv');
Hope this works out for you