use simple BBCode in your comments 
atm follow tags available:
[b]bold[/b] 
[i]italic[/i] 
[u]underline[/u] 
[color=color]text[/color] example [color=FF6600]Text[/color]
list 
[ul] 
[li]number 1[/li] 
[li]number 2[/li] 
[li]number 3[/li] 
[/ul]
[url=http://www.google.com]Google[/url] 
[img=http://uri.to.pic][/img] (fix picsize)
(required wp-syntax) 
[code]your code[/code], 
optional with declaration of the language 
[code lang=”csharp”]your sample code[/code] (all available languages from wp-syntax possible)
<h3>Arbitrary section</h3>
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BBComments

digitalpoint

The Shortcode Callback plugin allows you to use a [callback] shortcode to execute arbitrary PHP code wherever the shortcode is used.
Usage
Execute someFunction() and insert whatever it returns with the following shortcode: 
 [callback function=”someFunction”]
Example shortcode to include a PHP file (the path is relative to WordPress’ ABSPATH), then insert the results of someFunction() where you used the shortcode: 
 [callback include=”custom/filetoinclude.php” function=”someFunction”]
Shortcode example that includes a PHP file (the path is relative to WordPress’ ABSPATH), then passes a paramter to someFunction() and returns the results where you used the shortcode: 
 [callback function=”someFunction” include=”custom/filetoinclude.php” param=”something”]
The format to call a class/method with the shortcode is exactly the same as above, except you specify the class::method in the “function” attribute of the shortcode. 
 [callback function=”someClass::someFunction” include=”custom/filetoinclude.php” param=”something”]
There is an example (with PHP code) <a href="https://wordpress.org/plugins/shortcode-callback/faq/" rel="ugc">over here</a>.

Shortcode Callback

Adds a [callback] shortcode that can trigger PHP code so you can insert custom/complex things into your pages/posts.

BBComments vs Shortcode Callback