I have a textfield in a form that must contain a number. Using an integer for the field type is not possible - it must be text; however, I need to ensure that the input is actually a number. I've used:

function myfunc_form_user_register_form_validate($form, &$form_state) {
  if (!is_numeric($form_state['values']['number'])) {
    form_set_error('number', t('Field must be Numeric'));

However, it does not work, that is, no errors are ever displayed. I have implemented the above code in template.php. I know there are Form Validation modules; however, I would like to do such a simple thing myself. I also think it's a bit overkill to create a whole module to simply validate a single field in a single form! I also wonder how to find the $form[] keys. I named the input field number so I imagine that's the key but it's just a guess. I've looked through the Form API and it is not very clear what I am supposed to do. Simple things, such as finding out what's even in $form[] alludes me and it doesn't seem to be clearly explained.

  • How have you attached the validate handler to the form? Are you 100% sure myfunc_form_user_register_form_validate has been invoked?
    – Clive
    Commented Nov 5, 2015 at 13:04
  • 1) I assume so: $form['#validate'][] = "myfunc_form_user_register_form_validate"; 2) Yes, it is being called.
    – monster
    Commented Nov 5, 2015 at 14:19
  • I would check $form_state['values']['number'] in that case - as far as PHP is concerned, the string doesn't contain a valid number (that's the only way is_numeric would fail)
    – Clive
    Commented Nov 5, 2015 at 14:23
  • Sorry Clive, I made an error: the method is not being called! The form_alter method is running ($form['#validate'][] = "myfunc_form_user_register_form_validate";
    – monster
    Commented Nov 5, 2015 at 14:26
  • No worries, that's actually a more straightforward method to accomplish this anyway, I've chucked a quick answer in
    – Clive
    Commented Nov 5, 2015 at 14:27

2 Answers 2


Drupal already has a helper for this, in the shape of the element_validate_number() validation callback:

function MYMODULE_form_user_register_form_alter(&$form, &$form_state, $form_id) {
  $form['number']['#element_validate'][] = 'element_validate_number';

You can also use element_validate_integer() or element_validate_integer_positive if either of those make more sense for your use case.

  • Unfortunately, the method is calling; however, it is not validating. I think the problem could be number. I named the field number so I assume that's the key of the array? The fieldname is field_number. How do I know what key to use? Similarly, how do I find out other elements keys?
    – monster
    Commented Nov 5, 2015 at 14:31
  • I now get this error: Notice: Undefined index: #value in element_validate_number() (line 4385 of C:\xampp\htdocs\drupal\includes\form.inc).
    – monster
    Commented Nov 5, 2015 at 15:15
  • Oh right, yeah that would've been helpful to know ;) So it'll be more like $form['field_number'][LANGUAGE_NONE][0]['value']['#element_validate'][] = ...
    – Clive
    Commented Nov 5, 2015 at 15:37

You need to define your validate function in hook_form_<form_id>_alter($form, $form_state)

like the following

<themeName>_form_user_register_form_alter($form, $form_state) {
// your rest of the code
$form['#validate'][] = "myfunc_form_user_register_form_validate";

Clear the cache afterwards and check if the function is properly called

Hope this will help!

  • This seems to work. However, if (!is_numeric($form_state['values']['number'])) { in my method doesn't validate correctly. Instead I have to make it: is_numeric($form_state['values']['number']['und'][0]['value'])
    – monster
    Commented Nov 5, 2015 at 15:27

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