What is the difference between _form_validate vs form_validate?

Question

I'm unsure which one you should use for a form_validation.

Seeing that _form_validate() starts with a _ makes it a private function following the Drupal coding standards if I'm not mistaken.

So my guess is that a form validation should be done with hook_validate().

But the question remains: what's the difference between the two and where should they be used?

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EDIT: I misunderstood the hook_validate() which is not the _validate "hook" used in the form API. I'll leave the question unchanged so other users can learn from my mistake.

Answer 1

You're totally correct, Drupal's coding standards denote anything with a _ prefix is seen as a pseudo-'private' function. As such the _form_validate() function was never meant to be called directly. Unless you really need to use it, and you're absolutely positive there's no other way to accomplish what you want, I wouldn't advise using it.

In fact only one function in the whole core system calls _form_validate(): drupal_validate_form(). It would stand to reason that you should be using this function instead (being that it's not marked as a private function).

These are the functions that Drupal uses internally to validate a managed form, so if you're using the Form API correctly you should never need to call them. Adding validate handlers to a form to validate individual elements is the correct way to do things.

Just to clear up a wee bit of terminology as well, hook_validate() has nothing to do with the form API as such; it's used by node-providing modules to validate nodes before saving. The validation handlers that you add to forms (with custom forms, or through hook_form_alter(), don't have a particular 'Drupal name' that I'm aware of.

Also, you don't actually even need to use the standard named _validate function if you don't want to...you can add a function with any name as a form validate handler:

$form['#validate'][] = 'my_strangely_named_function';

Drupal just looks for FORMNAME_validate() by default, and runs it if it exists.

Answer 2

hook_validate performs node validation before a node is created or updated. This hook is invoked only on the module that defines the node's content type.

_form_validate checks if all required fields are filled and if max lengths of text field is not exceeded. If it finds any errors of such kind it fires form_error or form_set_error functions.

You need to use hook_validate when you define a new node's content type in your module and want to do some customized validation for this content type. This hook won't work for non-node forms.

Generally you won't need to call _form_validate, because it is being called automatically during form submission process.

The best practice to validate an arbitrary form is to create a custom validation function.

For example you have a function that defines your form:

function YOURFORM_form(){
    // form definition
}

The custom validation function for this form would be

function YOURFORM_validate(){
    // form validation
}

Answer 3

I suppose it's a custom form that you'd like to validate?

If your form function is called mymodule_form(), then you should add a function called mymodule_form_validate($form, &form_state). That function will be called automatically, and you'll be able to validate the submitted values using $form_state['values'].

If there are errors, you should call form_set_error(), which will prevent the submission and notify the user.

hook_validate() is only used to validate node submissions, if your module provides a content type, and can't be used to validate any other forms.