Field validation to compare two field values - php

Question

I've created a content type using two fields "capacity" and "count".
In a custom module I'd like to validate that field "count" should be less than field "capacity".
function MYMODULE_node_form_validate($form, &$form_state) {
$capacity = $form['capacity']['#value'];
$count = $form['count']['#value'];
if ($count > $capacity) {
form_set_error('title', 'Not possible');
}
}

Answer 1

First add a custom validation function, then fire your logic from there and prevent the form from further processing if necessary. Replace MYMODULE and MYCONTENTTYPE with your machine names.
/**
* Implements hook_form_BASE_FORM_ID_alter().
*/
function MYMODULE_form_node_form_alter(&$form, &$form_state, $form_id) {
// Find the content type of the node we are editing.
$content_type = $form['#node']->type;
if ($content_type == 'MYCONTENTTYPE') {
// Add an additional custom validation callback.
$form['#validate'][] = 'MYCUSTOM_FORMVALIDATION';
}
}
/**
* Custom MYCONTENTYTPE node form validation.
*/
function MYCUSTOM_FORMVALIDATION($form, &$form_state) {
// Better check isset() and !empty() first. Depends on your needs.
// Convert values to comparable numbers.
// Maybe you prefer intval() or some other logic. Depends on your needs.
$field_a = floatval($form_state['values']['field_a'][LANGUAGE_NONE][0]['value']);
$field_b = floatval($form_state['values']['field_b'][LANGUAGE_NONE][0]['value']);
// Stop the form from further processing if field A < than field B.
if ($field_a < $field_b) {
form_set_error('stop', t('Field A shall be greater then Field B'));
}
}