calculate xml size

["Vo, Lance" <lvo@ibsys.com>]

hi 
does php has any function that calculate a xml element in bytes or anything:
example:
 
<tests>
<test1>ABC</test1>
<test2>DEF</test2>
</tests>
 
If I want to calculate element <test1>, it should return 18 bytes.
Any suggestion is welcomed.
 
thanks
Lance