protected String convertKey(String key) {
        if (key != null) {
            return key.toLowerCase();
        } else {
            return null;
        }
    }
Converts keys to lower case.
<p/>
Returns null if key is null.

@param key the key convert
@return the converted key
jphp-runtime/src/php/runtime/common/collections/map/CaseInsensitiveMap.java