<?php

    function lineToHump($str)
    {
        return preg_replace_callback('/(_[a-z])/', function ($match) {
            return ucfirst(trim($match[0], '_'));
        }, $str);
    }

    function humpToLine($str)
    {
        return preg_replace_callback('/([A-Z])/', function ($match) {
            return '_' . lcfirst($match[0]);
        }, $str);
    }
    echo lineToHump("new_id");
    echo "\n";
    echo humpToLine("newId");

newId
new_id