Specify the format of the syslog entry. Available variables are: <dl><dt><code>!base_url</code></dt><dd>Base URL of the site.</dd><dt><code>!timestamp</code></dt><dd>Unix timestamp of the log entry.</dd><dt><code>!type</code></dt><dd>The category to which this message belongs.</dd><dt><code>!ip</code></dt><dd>IP address of the user triggering the message.</dd><dt><code>!request_uri</code></dt><dd>The requested URI.</dd><dt><code>!referer</code></dt><dd>HTTP Referer if available.</dd><dt><code>!severity</code></dt><dd>The severity level of the event; ranges from 0 (Emergency) to 7 (Debug).</dd><dt><code>!uid</code></dt><dd>User ID.</dd><dt><code>!link</code></dt><dd>A link to associate with the message.</dd><dt><code>!message</code></dt><dd>The message to store in the log.</dd></dl>