This is a quick one, thanks to Drupal’s Form API!
Use #attributes to set a placeholder or other attributes you need on an input.
function mymodule_test_form($form, &$form_state) {
$form = array();
$form['name'] = array(
'#type' => 'text',
'#title' => 'Your Name',
'#attributes' => array(
'placeholder' => 'First Name',
'data-tooltip' => 'Enter your first name here',
),
);
$form['submit'] = array(
'#type' => 'submit',
'#value' => 'Submit',
);
return $form;
}