Change theme skin
In case you would like to have the latest theme for bootstrap version 4 then you will simply need to add the below line of code at your PHP CRUD:
$crud->setSkin('bootstrap-v4');
Example
Below you can find a full working example with bootstrap version 3 theme:
$crud->setSkin('bootstrap-v4');
$crud->setTable('customers');
$crud->setSubject('Customer', 'Customers');
$crud->columns(['customerName','phone','addressLine1','creditLimit']);
$output = $crud->render();
You can see the results of the above code below: