Add the ability to disable the PHP Code execution feature

To disable the "PHP Code" execution feature, add the following code your theme's functions.php file:

add_filter('post_snippets_php_execution_enabled', '__return_false');

Doing so will:
- remove the PHP Code checkbox from the post snippets settings screen
- remove the PHP code contextual help
- ensure that PHP code cannot be executed in a post snippet

As per this suggestion: http://wordpress.org/support/topic/disabling-php-code-functionality