How to disable, hide or lock a form field in Drupal

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First Create a function hook_form_alter.Then inside hook_form_alter depending upon your choice :
a) If you want to disable field (it will be visible, but user can't change it) :

$form['field_myfield']['#disabled'] = TRUE;

b) If you want it to be hidden, but to keep value it has before editing the way to do that is:

$form['field_myfield']['#access'] = FALSE;

c) if you want to hide it:

hide($form['field_myfield']);