Symfony 3 - modify form after getForm - php

Question

I have a form builder set to create an user info form:
$form = $this->createFormBuilder($userSettings)
->add('newName', TextType::class, array(
'required' => false,
'label' => "Change UserName",
'attr' => array(
'placeholder' => $userData[0]->getUsername()
)
))
->add('newMail', TextType::class, array(
'required' => false,
'label' => "Change Email",
'attr' => array(
'placeholder' => $userData[0]->getEmail()
)
))
->add('newPassword', PasswordType::class, array(
'required' => false,
'label' => "Change Password"
))
->add('currentPassword', PasswordType::class, array(
'label' => "Current Password"
))
->add('save', SubmitType::class, array('label' => 'Update Information!'))
->getForm();
Now after form is submitted I have a handler which would check if the mail or username are not already taken
// Check if name or mail not occupied
if($user_manager->findUserByEmail($mail)){
}
elseif($user_manager->findUserByUsername($name)){
}
My goal is to, on mail/username occupied to have the form be printed again, only this time, occupied values would have a placeholder saying "THIS VALUE IS OCCUPIED"
Now I have tried to add another "add" into the if statement like
$form->add('newMail', TextType::class, array(
'required' => false,
'label' => "Change Email",
'attr' => array(
'placeholder' => "EMAIL TAKEN"
)
));
Unfortunately this returns me an error saying
You cannot add children to a submitted form
Now here is my question:
Is there a good and proper way to do it?
B-Side:
Could I trigger browser default validation like ie. In chrome if an input is required and you fail to put stuff there, the box would turn red and you would get an error message.
I was wondering if I could do the same here, so for this project so in that case I would get a red input box with alert "Email taken" or something like that

Answer 1

maybe something like this?
$form = $this->createFormBuilder($userSettings)
->add('newName', TextType::class, array(
'required' => false,
'label' => "Change UserName",
'attr' => array(
'placeholder' => $userData[0]->getUsername()
)
))
->add('newMail', TextType::class, array(
'required' => false,
'label' => "Change Email",
'attr' => array(
'placeholder' => $userData[0]->getEmail()
)
))
->add('newPassword', PasswordType::class, array(
'required' => false,
'label' => "Change Password"
))
->add('currentPassword', PasswordType::class, array(
'label' => "Current Password"
))
->add('save', SubmitType::class, array('label' => 'Update Information!'));
if(isset($mail) && $user_manager->findUserByEmail($mail)){
$form->remove('newMail')
->add('newMail', TextType::class, array(
'required' => false,
'label' => "Change Email",
'attr' => array(
'placeholder' => 'EMAIL TAKEN'
)
));
}
elseif(isset($name) && $user_manager->findUserByUsername($name)){
// just duplicate the above
}
$form->getForm();