replace all weird characters in xml to its equivalent 'character reference'

I have an XML which has some characters like,

£ °                   /* entity reference aka symbolic character ref */
» & Â • ›                       /* unicode characters */

I want to replace both such characters (entity ref & unicode chars) with their "character references" aka "numeric character ref",

 £  - character reference for '£' OR '£'

Is there a php function which does this?

Thanks!