It's a very simple function, but I'm going to be picky (it's late here and this is what I do when I'm tired).

• Why do you create $szOutput? It is assigned a value and immediately the next line is the return.
• Speaking of the return, why is $szOutput trimmed just before returning if the string is limited, but not in any other case?
• This is my main gripe, the actual length of the returned value (if the string has been trimmed) is $iLength + strlen($szAffix). The returned string should be of length $iLength, not greater. What's the point of limiting to, say, 20 characters when the actual string returned is 23?
• The function, as is, has a syntax error -- line 6, the last parenthesis shouldn't be there.