With this plugin you can include whatever file you want in a post or in a page. So you can write PHP functionalities and make them available in a page. When you will change the template nothing bad happens.
Download the plugin here: Include 1.0 (29)
Example of including the readme.txt of this plugin:
Warning: include_the_content(/home/mhd-01/www.satollo.com/htdocs/wp-content/plugins/include/readme.txt) [function.include-the-content]: failed to open stream: No such file or directory in /home/mhd-01/www.satollo.com/htdocs/wp-content/plugins/include-it/plugin.php on line 86
Warning: include_the_content() [function.include]: Failed opening '/home/mhd-01/www.satollo.com/htdocs/wp-content/plugins/include/readme.txt' for inclusion (include_path='./:/usr/share/php/') in /home/mhd-01/www.satollo.com/htdocs/wp-content/plugins/include-it/plugin.php on line 86
