Hook form alter to check date field is empty or not

Question

I'm using hook_form_alter() to add some custom validations. I have to check if a date field is empty or not; when the field is empty, I need to display an error message.

I don't know how to get the value from date field after a form is submitted and check it with empty().

I have tried the following lines, but none of them worked.

• $form_state['input']['field_start_date']['und'][0]['value']
• $form['field_start_date']['und'][0]['#default_value']['value']
• $form_state['values']['field_start_date']

What am I doing wrong?

Answer 1

If the form contains a field_start_date form element, $form_state['values']['field_start_date'] is the only correct way to get the submitted values for that field.
Neither $form_state['input']['field_start_date']['und'][0]['value'] nor $form['field_start_date']['und'][0]['#default_value']['value'] is the correct way to fetch the submitted values.

Keep also in mind that, if you need to show a validation error when the value submitted for a form element is an empty value, probably you just need to set that form element as required. In that way, Drupal would require users enter a value and show a validation error to the users who don't enter a value. (Drupal would also show a red asterisk close to the form element, to let users know that form element is required.)
For this, it's enough to set #required to TRUE in hook_form_alter(). It doesn't require to write validation code, as that is done from Drupal core.