camelize.php

<?php
function camelize($input, $lower = false)
{
    $result    = '';
    $words     = explode('_', $input);
    $wordCount = count($words);
    for ($i = 0; $i < $wordCount; $i++) {
        $word = $words[$i];
        if (!($i === 0 && $lower === false)) {
            $word = ucfirst($word);
        } else {
            $word = strtolower($word);
        }
        $result .= $word;
    }
    return $result;
}

$input = "camel_case_string";
echo camelize($input) . PHP_EOL;       // => CamelCaseString
echo camelize($input, true) . PHP_EOL; // => camelCaseString