Change CamelCaseString to underscore_style_string

gistfile1.php

/**
 * Changes a CamelCaseString into a string_with_underscores.
 *
 * @param string $string CamelCase string
 * @return string string_with_underscores
 */
public static function camelCaseToUnderscoreStyle($string)
{
    return preg_replace_callback(
        '|[A-Z]|',
        function ($match) {
            return '_' . strtolower($match[0]);
        },
        lcfirst($string)
    );
}

Taking into account unicode support.
http://phpfiddle.org/main/code/sy15-vtqp

/**
 * Zmieniamy string CamelCase na pisane_z_podkresleniami.
 *
 * @param string $string String CamelCase
 * @return string string_z_podkresleniami
 */
public static function camelCaseToUnderscoreStyle($string)
{
    // Default patterm
    $pattern = array('#(?<=(?:[A-Z]))([A-Z]+)([A-Z][A-z])#', '#(?<=(?:[a-z0-9]))([A-Z])#');

    // Check fo unicode support
    $unicodeSupportEnabled = (@preg_match('/\pL/u', 'a')) ? true : false;

    if ($unicodeSupportEnabled) {
        // Change pattern for unicode support
        $pattern = array('#(?<=(?:\p{Lu}))(\p{Lu}\p{Ll})#','#(?<=(?:\p{Ll}|\p{Nd}))(\p{Lu})#');
    }

    return preg_replace_callback(
        $pattern,
        function ($match) {
            return '_' . strtolower($match[0]);
        },
        lcfirst($string)
    );
}