I use this function very rarely. For some reason I just never need to use it. I know in Drupal it is used quite often, especially if you use the PHP input field for a node. This allows you to enter PHP code that will be evaluated (or run) and then the output is shown as the node’s content.

I suppose this is mostly used when you want a user to be able to enter PHP code into a text field and then run it. Not always too secure I would think, so be careful where and how you use this.

  $s = "<?php phpinfo(); ?>";
  eval($s);