PHP Was originally a meta-syntax that was parsed and simply dynamically called-into a compiled c-programs functions.
In this way you can think of it similar to a "shell" program for the "web"...
It's no wonder variables with $dollarsigns were used similar to shell variables.
When I was all of 12, and I was learning PHP 3... as my first programming language - having the $ made life so much easier. I knew what a variable was, but having a reminder in all of the source code I read made learning possible... not sure I'd be here today if I didn't start on PHP.
Here's an example from -> http://www.php.net/manual/en/history.php.php
<!--ifsubstr $exec_result Mozilla-->
Hey, you are using Netscape!<p>
<!--sql database select * from table where user='$username'-->
<!--ifless $numentries 1-->
Sorry, that record does not exist<p>
You have <!--$index:0--> credits left in your account.<p>