Server Side Includes: variable in <!--#if not found, #echo prints content

Question

I want to check if a file exists with <!--#if expr='-F %{foo}' --> but apaches error.log says the variable wouldn't exist. <!--#echo var="foo" --> works fine.

I started in a file with hard coded values where it worked fine. Now I want to create a parameterized snippet I can include in several other files.

<!--#set var="foo" value="/path/to/file" -->

<!--#echo var="foo" --><br>

<!--#if expr='-F %{foo}' -->
exists<br>
<!--#else -->
doesn't exist<br>
<!--#endif -->

<!--#echo var="foo" -->

I'd expect the output

/path/to/file
exists
/path/to/file

but instead I get the error message saying that the variable wouldn't exist in apaches error.log:

AH01337: Could not parse expr "-F %{foo}": Variable 'foo' does not exist

_{it's not so 01337 if you'd ask me...}

Answer 1

Having a look at the mod_include documentation about flow control and the Apache Expressions Syntax shows two things which might interfere here and cause this issue:

• The examples in the mod_include documentation about flow control use the v("foo") syntax and not the %{foo} syntax to refer to variables (unless SSILegacyExprParser is set to on, but the legacy syntax seems to be $foo anyways).
• The Apache Expressions Syntax suggests that only a fixed list of server-set variables are usable with the %{…} syntax.

So I assume that there are two different types of variables and only one (the server-provided variables) can be used with the %{…} syntax. (And yes, these different ways to refer to a variable look confusing to me, too.)