Fatal error: Call to undefined function ctools_include()  While implementing Drupal 6 Multistep Form Using Chaos Tools I would like to implement a multi-step user registration form. After a long time googling, I found Intro to Drupal 6 Multistep Form Domination Using Chaos Tools. While I was trying to implement the suggested code, I got the following error: 


  Fatal error: Call to undefined function ctools_include() in var/www/WC/drupal_website/sites/all/modules/wombat/wombat/wombat.module on line 26


How do I solve it?
As the Chaos tools suite module defines ctools_include() in ctools.module, the only reason I can think of what would explain the error is that you didn't install the module. The other reason could be that you are trying to access the function when the module is still not loaded; for example, if you call the function inside hook_boot(), then you would get that error.