Use $_POST data in functions.php

Try adding you query variable to the public query variable list like this.

function wp280272_query_vars($vars) {
  $vars[] = 'currency';
  return $vars;
}
add_filter( 'query_vars', 'wp280272_query_vars' );

you then can access it via

get_query_var('currency');

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)