Wordpress - Include PHP file in Content using [shortcode]

Here's another way to do it, utilizing get_template_part of wordpress

function include_file($atts) {
     $a = shortcode_atts( array(
        'slug' => 'NULL',
       ), $atts );

      if($slug != 'NULL'){
        ob_start();
        get_template_part($a['slug']);
        return ob_get_clean();
      }
 }
 add_shortcode('include', 'include_file');

examples:

[include slug="form"]

[include slug="sub-folder/filename_without_extension"]

I modified some code from an old blog post to do this, and allow for query-strings to be attached to the file as well.

Original credit goes to amberpanther.com, and it turns out they made a plug-in out of this, too.

//create the shortcode [include] that accepts a filepath and query string
//this function was modified from a post on www.amberpanther.com you can find it at the link below:
//http://www.amberpanther.com/knowledge-base/using-the-wordpress-shortcode-api-to-include-an-external-file-in-the-post-content/
//BEGIN amberpanther.com code
function include_file($atts) {
     //if filepath was specified
     extract(
          shortcode_atts(
               array(
                    'filepath' => 'NULL'
               ), $atts
          )
     );

     //BEGIN modified portion of code to accept query strings
     //check for query string of variables after file path
     if(strpos($filepath,"?")) {
          $query_string_pos = strpos($filepath,"?");
          //create global variable for query string so we can access it in our included files if we need it
          //also parse it out from the clean file name which we will store in a new variable for including
          global $query_string;
          $query_string = substr($filepath,$query_string_pos + 1);
          $clean_file_path = substr($filepath,0,$query_string_pos);     
          //if there isn't a query string
     } else {
          $clean_file_path = $filepath;
     }
     //END modified portion of code

     //check if the filepath was specified and if the file exists
     if ($filepath != 'NULL' && file_exists(get_stylesheet_directory_uri() . "/" . $clean_file_path)){
          //turn on output buffering to capture script output
          ob_start();
          //include the specified file
          include(TEMPLATEPATH.$clean_file_path);
          //assign the file output to $content variable and clean buffer
          $content = ob_get_clean();
          //return the $content
          //return is important for the output to appear at the correct position
          //in the content
          return $content;
     }
}
//register the Shortcode handler
add_shortcode('include', 'include_file');
//END amberpanther.com code
//shortcode with sample query string:
//[include filepath="/get-posts.php?format=grid&taxonomy=testing&term=stuff&posttype=work"]

I set mine to pull from the stylesheet uri (so it will work with child themes and such), but you could adjust that code easily to pull from anywhere (including plug-in directories), or remove it altogether and just use the full path when including the file. You could even add a conditional with a trigger character at the beginning that tells it to use a specific path based on what the first character of the file name is, like using a "#" for the template directory, etc.

I use it to pull in a file called get-posts.php that lives in my template directory and formats the output of various posts based on a series of parameters provided in the query string. It saves me from needing special templates because I can include the file, send it a format and it will output the posts with the markup I've specified in the get-posts.php file.

It also allows clients to pull custom post types into actual blog-posts in specific formats and is quite handy.

Let me know if you need clarification on anything.

There is an error in the solution provided by @adedoy, since $slug is never defined. This worked for me:

function include_file($atts) {

$atts = shortcode_atts(
    array(
    'path' => 'NULL',
    ), $atts, 'include' );

    ob_start();
    get_template_part($atts['path']);
    return ob_get_clean();

}

add_shortcode('include', 'include_file');

Usage: [include path="path/from/root"]