strtoupper and HTML entities

Hello, I know I can convert a string to all uppercases with the strtoupper() function. However, what if the original string contains HTML entities? EXAMPLE: original string = "funkstörung" contains lowercase o with dieresis (ö) properly displayed by browsers after strtoupper becomes: "FUNKSTÖRUNG" which is displayed by browsers literally, because capital O with dieresis is Ö The uppercase version correctly displayable in a browser should be: "FUNKSTÖRUNG How can I convert to uppercase everything except these entities? Is there a straightforward solution, or do I really put together a regex that puts in lowercase everything from the second letter after an ampersand to the first colon? Thanks, mweb