PHP code help!

Thank you for the great module!
I want to validate a field input in a node edit form against values already stored for this field in the database. The field is a nodereference.
So I use this php code:

/**
*Implement a function to get the NID of the currently edited node.
**/
function node_get_id(){
if(arg(0) == 'node' && is_numeric(arg(1))){
$nid = arg(1);
$node = node_load(array('nid'=> $nid));
   }
}
/**
*Implement a function to find the ID of the referenced node 
*by the nodereference field in the currently edited node  
**/
function field_get_id() {
$currentnodeid = node_get_id();
$query = db_select('field_data_field_period_1', 'f')
    ->fields('f', 'field_period_1_nid')
    ->condition('entity_id', $currentnodeid, '=')
    ->execute()
    ->fetchField();
}
/**
*Implement a function to find the TITLE of the referenced node, 
*which is the actual value of the validated field   
**/
function field_get_title() {
$reffieldid = field_get_id();
$query = db_select('node', 'n')
    ->fields('f', 'title')
    ->condition('nid', $reffieldid, '=')
    ->execute()
    ->fetchField();
}
/**
*Function to validate the new input value against the one stored in the database 
*and set error if there is already a value and it is different from the new user input
**/
function validate_input_value() {
$fieldcurrvalue = field_get_title();
if(isset($fieldcurrvalue)&&($this->value != $fieldcurrvalue)){
  $this->set_error();
}
}

But nothing happens at all. I wonder what my mistake is?