- var_dump: documentation ( source)
- preg_replace_callback: documentation ( source)
<?php
echo preg_replace_callback('/([a-z])([A-Z])/', '$1_$2', 'eOaUi');
echo preg_replace_callback('/([a-z])([A-Z])/', function (array $matches) {
var_dump($matches);
}, 'eOaUi');