Modules/Doc: Add Syntax highlighting for PHP code

refs #4820

doc/CONFIG.md

@@ -8,6 +8,7 @@ INI files as source it enables you to store configuration in a familiar format.
 defines some configuration files for its own purposes. Please note that both modules and framework
 keep their main configuration in the INI file called config.ini. Here's some example code:
 
+```php
 <?php
 use \Icinga\Application\Config as IcingaConfig;
 
@@ -23,6 +24,7 @@ keep their main configuration in the INI file called config.ini. Here's some exa
 // the module's config.ini. For using other files you have to pass this parameter though.
 // The following example loads values from the example module's extra.ini:
 IcingaConfig::module('example', 'extra')->logging->get('enabled', true);
+```
 
 ## Reload from disk
 

modules/doc/library/Doc/DocParser.php

@@ -126,9 +126,26 @@ class DocParser
             $fileObject->flock(LOCK_UN);
         }
         $html = Parsedown::instance()->parse(implode('', $cat));
+        $html = preg_replace_callback(
+            '#<pre><code class="language-php">(.*?)\</code></pre>#s',
+            array($this, 'highlight'),
+            $html
+        );
         return array($html, $toc[0]->item);
     }
 
+    /**
+     * Syntax highlighting for PHP code
+     *
+     * @param   $match
+     *
+     * @return  string
+     */
+    protected function highlight($match)
+    {
+        return highlight_string(htmlspecialchars_decode($match[1]), true);
+    }
+
     /**
      * Extract atx- or setext-style headers from the given lines
      *