Convert under_score type array's keys to camelCase type array's keys and likewise

transformKeys.php

<?php

    /**
     * Convert under_score type array's keys to camelCase type array's keys
     * @param   array   $array          array to convert
     * @param   array   $arrayHolder    parent array holder for recursive array
     * @return  array   camelCase array
     */
    public function camelCaseKeys($array, $arrayHolder = array()) {
        $camelCaseArray = !empty($arrayHolder) ? $arrayHolder : array();
        foreach ($array as $key => $val) {
            $newKey = @explode('_', $key);
            array_walk($newKey, create_function('&$v', '$v = ucwords($v);'));
            $newKey = @implode('', $newKey);
            $newKey{0} = strtolower($newKey{0});
            if (!is_array($val)) {
                $camelCaseArray[$newKey] = $val;
            } else {
                $camelCaseArray[$newKey] = $this->camelCaseKeys($val, $camelCaseArray[$newKey]);
            }
        }
        return $camelCaseArray;
    }

    /**
     * Convert camelCase type array's keys to under_score+lowercase type array's keys
     * @param   array   $array          array to convert
     * @param   array   $arrayHolder    parent array holder for recursive array
     * @return  array   under_score array
     */
    public function underscoreKeys($array, $arrayHolder = array()) {
        $underscoreArray = !empty($arrayHolder) ? $arrayHolder : array();
        foreach ($array as $key => $val) {
            $newKey = preg_replace('/[A-Z]/', '_$0', $key);
            $newKey = strtolower($newKey);
            $newKey = ltrim($newKey, '_');
            if (!is_array($val)) {
                $underscoreArray[$newKey] = $val;
            } else {
                $underscoreArray[$newKey] = $this->underscoreKeys($val, $underscoreArray[$newKey]);
            }
        }
        return $underscoreArray;
    }

    /**
     * Convert camelCase type array's values to under_score+lowercase type array's values
     * @param   mixed   $mixed          array|string to convert
     * @param   array   $arrayHolder    parent array holder for recursive array
     * @return  mixed   under_score array|string
     */
    public function underscoreValues($mixed, $arrayHolder = array()) {
        $underscoreArray = !empty($arrayHolder) ? $arrayHolder : array();
        if (!is_array($mixed)) {
            $newVal = preg_replace('/[A-Z]/', '_$0', $mixed);
            $newVal = strtolower($newVal);
            $newVal = ltrim($newVal, '_');
            return $newVal;
        } else {
            foreach ($mixed as $key => $val) {
                $underscoreArray[$key] = $this->underscoreValues($val, $underscoreArray[$key]);
            }
            return $underscoreArray;
        }
    }

Would sugget replacing:
array_walk($new_key, create_function('&$v', '$v = ucwords($v);'));
With

array_walk($new_key, function (&$v){ $v = ucwords($v); });

Many Thanks @nscheuner

This function that worked for me:
(and works with array of arrays)

function camelCaseKeys($array)
{
    foreach($array as $key => $value) {
        if (is_array($value)) {
            foreach($value as $x => $y) {
                $camelCase = str_replace(' ', '', ucwords(str_replace('_', ' ', $x)));
                $camelCase[0] = strtolower($camelCase[0]);
                $array[$key][$camelCase] = $y;
                unset($array[$key][$x]);
            }
        } else {
            $keyCamelCase = str_replace(' ', '', ucwords(str_replace('_', ' ', $key)));
            $keyCamelCase[0] = strtolower($keyCamelCase[0]);
            $array[$keyCamelCase] = $value;
            unset($array[$key]);
        }
    }

    return $array;
}

@flads your function is close, but ID becomes iD