Which Drupal functions can be called in hook_requirements('install')? Generally, implementations of hook_requirements() don't use any Drupal function, when they are invoked during installation. node_requirements() uses some Drupal functions for whatever phase it is invoked.

  $requirements = array();
  // Ensure translations don't break at install time
  $t = get_t();
  // Only show rebuild button if there are either 0, or 2 or more, rows
  // in the {node_access} table, or if there are modules that
  // implement hook_node_grants().
  $grant_count = db_query('SELECT COUNT(*) FROM {node_access}')-&gt;fetchField();
  if ($grant_count != 1 || count(module_implements('node_grants')) &gt; 0) {
    $value = format_plural($grant_count, 'One permission in use', '@count permissions in use', array('@count' =&gt; $grant_count));
  }
  else {
    $value = $t('Disabled');
  }


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));
}