First off... let's clean things up a bit... try this:
$fields = array(
'username' => 'please enter a username',
'firstname' => 'please enter your first name',
'lastname' => 'please enter your last name',
'email' => 'please enter your email address',
'mobile' => 'please enter your mobile number',
'country' => 'please enter your country',
'password1' => 'please enter a password',
'password2' => please re-enter your password',
);
foreach ($fields as $field => $value) {
if ("" == $value) { $error = $fields[$field]; }
}
This will neaten things up a bit. Also try this for the $query:
$query = sprintf("SELECT username FROM %s WHERE username = '%s'",c_reg_users,$username);
Then run the query.