Which Drupal functions can be called in hook_requirements('install')?
I've made severals tests and I found that depending on modules that you've installed or other things the list of available functions is quite different.

So, IMHO the best way to see which functions you can access at that moment its to list them with the PHP function get_defined_functions. Below it's the small code that I used to list all the functions you can use.

&lt;?php
if ($phase == 'install') {
  $funcs = get_defined_functions();
  file_put_contents('./functions.txt', var_export($funcs['user'], 1));
}