Convert Camel Case ↔ Snake Case

<?php
function camelToSnake(string $input): string {
    $out = preg_replace('/(?<!^)[A-Z]/', '_$0', $input);
    return strtolower($out);
}

function snakeToCamel(string $input, bool $upperFirst = false): string {
    $parts = explode('_', strtolower($input));
    $head  = $upperFirst ? ucfirst($parts[0]) : array_shift($parts);
    $rest  = array_map('ucfirst', $parts);
    return $upperFirst ? $head . implode('', $rest) : $head . implode('', $rest);
}

echo camelToSnake('userFirstName');         // user_first_name
echo snakeToCamel('created_at');            // createdAt
echo snakeToCamel('foo_bar_baz', true);     // FooBarBaz