HTTP 500 - Internal Server Error While posting Link

[devoidfeast]

Hello Fellows,
When i try to post a new link i get "HTTP 500 - Internal Server Error" as shown in the image below
Attachment 149624

Here is the Apache error log

"[Sun Jul 06 02:16:09 2014] [warn] Cannot get media type from 'x-httpd-php'
[Sun Jul 06 02:58:10 2014] [error] [client xxx.xxx.xxx.xxx] PHP Fatal error: Class 'DOMDocument' not found in /var/www/forum/core/vb/api/content/link.php on line 120, referer: http://mydomain.com/new-content/19
[Sun Jul 06 02:58:17 2014] [warn] Cannot get media type from 'x-httpd-php'
[Sun Jul 06 02:58:23 2014] [error] [client xxx.xxx.xxx.xxx] PHP Fatal error: Class 'DOMDocument' not found in /var/www/forum/core/vb/api/content/link.php on line 120, referer: http://mydomain.com/new-content/19"

If i post a thread with out a link it works perfect.but whenever i include a link it causes a problem.
I removed the .htaccess file and renamed the htaccess.txt file but it still through me the same error.there is no mod_rewrite issue it is enabled.
How can i resolve this issue.
Thank you