Enable php functions to xsl format

(Jira is down so i can't open an issue request)

Idea is simple: extends xsl format in order to inject some php class/method on different tags. For example: i would like to add syntax color on literal tag.

An example:
```
<!-- xsl:
<xsl:value-of select="php:function ('My\Name\Space\MyClassName::methodName',1+2)"/>
--->
```
```php
<?php
namespace My\Name\Space;

   class MyClassName
   {
      static public function methodName($someArgument)
      {
         return "resultis:".$someArgument;
      }
   }
?>
```
```
<!-- result:
resultis:3
-->
```

What is your opinion ?
Cheers.