echo string with question mark in it

["Brent Clements" <bclem@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

I have the following string variable

$string = 'test.php?id=' . $id;

but when I echo out the string it looks like this

test.php?
id=######### 

####### is the $id variable value btw.

How do I make the ? part of the string instead of php evaluating that question mark?

I've tried adding \ before the ? but it doesn't work.

Thanks,
Brent