The article Tcl the misunderstood does a good job explaining most of Tcl's features. But some things can shock most programmers. Things that could fulfill the perverted dreams of a PHP user:
$ tclsh
% set a pu
% set b ts
% $a$b "Hello World"
Hello World

But don't let this stop you from learning/using Tcl.It's still a nice glue language.