If your web host prevents WordPress from opening files in /tmp/ (a common default temp directory for PHP), then you may receive an error like the following:

[10-Mar-2015 07:04:35 UTC] PHP Warning: fopen(/tmp/comet-cache-54843.lock): failed to open stream: Permission denied in /var/www/example.com/wp-content/plugins/comet-cache-pro/includes/share.php on line 1938 [10-Mar-2015 07:04:35 UTC] PHP Fatal error: Uncaught exception 'Exception' with message 'Unable to obtain an exclusive lock.' in /var/www/example.com/wp-content/plugins/comet-cache-pro/includes/share.php:1939

To fix this, you can define a custom temp directory specifically for WordPress and then give that custom temp directory write permissions (e.g., 755 or 777).

Creating and defining a custom WordPress temp directory

  1. Create a custom temp directory, for example wp-content/temp/
  2. Give that custom temp directory permission for WordPress to write to (e.g., 755 or 777)
  3. Tell WordPress to use your custom temp directory by adding the following to wp-config.php:

Note: Replace /home/username/www/wp-content/temp with the full path to the temp directory on your server. Also, note that the line should be added above the line that says /* That's all, stop editing! Happy blogging. */

You can read more on the WordPress Codex about Changing File Permissions. If you need help creating the temp directory, setting file permissions, or updating your wp-config.php file, please contact your web host.