When the scripting engine encounters a left brace ({) followed by a variable name, it assumes the brace is associated with the variable, and absorbs it and any matching right brace into the variable.

That is,

<?php echo "{$Hello}"; ?>

will produce "" as output, not "{}".  Also, 

<?php echo "{$Hello there}"; ?>

will produce an error:

Parse error: parse error, expecting `'}'' in /ch/admin.rt/www.admin-dev/html/home/test/test3.cgi on line 2

rather than the expected "{ there}".  

I presume this has to do with the improved variable referencing in quoted strings.  I noticed this in the context of the IMAP extension, which actually uses curly braces in its function arguments (see imap_open as an example).