cms-niceties/src/Extensions/PlaceholderFormExtension.php

<?php

namespace A2nt\CMSNiceties\Extensions;

use SilverStripe\Core\Extension;
use SilverStripe\Forms\CompositeField;
use SilverStripe\Forms\FieldList;
use SilverStripe\Forms\TextField;

class PlaceholderFormExtension extends Extension
{
    public function updateFormFields(FieldList $fields)
    {
        foreach ($fields as $field) {
            $this->setPlaceholder($field);
        }
    }

    private function setPlaceholder($field)
    {
        if (is_a($field, TextField::class)) {
            if (!$field->getAttribute('placeholder')) {
                $placeholder = $field->Title() .($field->hasClass('requiredField') ? '*' : '');

                $field->setAttribute(
                    'placeholder',
                    $placeholder
                );
                $field->setTitle('');
            }
        }

        if (is_a($field, CompositeField::class)) {
            $children = $field->getChildren();
            foreach ($children as $child) {
                $this->setPlaceholder($child);
            }
        }
    }
}