php regular expression for unicode

I have some text where unicode is written as text l开发者_JAVA百科ike this

There areu25ba 2 boys.

it should be like this

There are&#x25ba 2 boys.

Replace 'u' with '&#x' if there is unicode character.

Thanks in advance

A naive way would be:

preg_replace('/u([0-9a-fA-F]{4})/','&#x$1;',$string);

But I doubt 'udaff' would appreciate it.