/usr/share/php/Symfony/Bundle/FrameworkBundle/Resources/views/Form/checkbox_widget.html.php is in php-symfony-framework-bundle 2.7.10-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | <input type="checkbox"
<?php echo $view['form']->block($form, 'widget_attributes') ?>
<?php if (strlen($value) > 0): ?> value="<?php echo $view->escape($value) ?>"<?php endif ?>
<?php if ($checked): ?> checked="checked"<?php endif ?>
/>
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