what does "$" mean in this SSI conditional statement?

Question

I was reading about SSI here. The first code example under the section Control Directives looks like this:

<!--#if expr="${Sec_Nav}" -->
<!--#include virtual="" -->
<!--#endif -->

What does the "$" sign mean? EOF, a function, get...???

Thanks!

Answer 1

It is just a way to inject a variable value inside a string

<!--#if expr="${Sec_Nav}" -->

this is comparing expr to the value of Sec_Nav