How can I create a variable with different values for each domain using Domain Access?
You must enable "domain configuration" submodule (domain_conf)
This will provide you the save per domain option:

function your_module_form($form, &amp;$form_state) {

  $form['variable'] = array(
    '#type' =&gt; 'value',
    '#value' =&gt; time(),
  );

  return system_settings_form($form);

}


getting the variable manually:

function domain_conf_variable_get($domain_id, $variable = '', $all = FALSE, $reset = FALSE)


seeting the variable manually:

function domain_conf_variable_set($domain_id, $variable, $value = NULL) 


There is a module which aims to be a replacement variable handling module for Domain Access,If you need setting different variable values for each domain and for each language at the same time you need domain variable module https://drupal.org/project/domain_variable