drupal form submission
There are a few things wrong with your approach, don't use $_POST, the values are there in &amp;$form_state and you will break Drupal form handling using $_POST. If you have devel running use dpm() to explore that array. 

In the theme function call drupal_get_form with the right name: forexCompare_form, i.e. 
drupal_get_form('forexCompare_form'). Or was the theme function just an example.