** Using WeBWorK **

FIrst, note that BEGIN_TEXT/END_TEXT (which I will call TEXT for brevity) works very differently from BEGIN_PGML/END_PGML (which I will refer to as PGML), as described below.

In TEXT mode, the text string is processed by a series of regular expression substitutions. First command substitution is performed (so \{...\} is replaced by the result of running to code between the delimiters), then variable substitution is performed (where $var is replaced by its contents), and finally LaTeX math is processed (so \(...\) and \[...\] are repaved by the typeset mathematics in whatever form is currently selected by the user, e.g., Images, MathJax, etc.).

What this means is that the substitutions later in the list can act on the results of the earlier substitutions. So in your example, your original text is

\{ $altTitle \}
$BR$BR

alt="$setNumber:$questionNumber"
$BR$BR

At this point, variable substitution is performed, and so all four variables are substituted into the string. The result will be something like

alt="3:7"
<br/><br>

Note that this relies on your having used command substitution to perform the initial including of $altTitle. Had you used normal variable substitution, as in

$altTitle
$BR$BR

alt="$setNumber:$questionNumber"
<br/><br>

As an aside, note that the reverse is not possible: you can't get commands performed in the results of variable substitutions (since command substitution is performed before variable substitution). So

$cmd = '\{ BeginTable().AlignRow(["A","B"]).EndTable() \}';

BEGIN_TEXT
$cmd
END_TEXT

$cmd = '\{ BeginTable().AlignRow(["A","B"]).EndTable() \}';

BEGIN_TEXT
\{$cmd\}
END_TEXT

PGML, on the other hand, does not do iterated regular expression substitution, but instead actually parses the string and performs actions on the parts in a programmatic way. That means that the results of a substitution are not affected by later substitutions (there are no later substitutions). Your use of [@ $altTitle @] rather than the more natural (and completely equivalent) [$altTitle] suggests that you are trying to apply the TEXT substitution paradigm incorrectly to PGML, which doesn't work that way.

The four forms of substitution (with different numbers of stars) work as follow:

• With no stars, the value of $altTitle are inserted into the resulting output with any special characters escaped so that they show up properly in whatever output mode you are using. That is, you should see exactly on screen or in hardcopy the characters that are in the content of the variable. In this case, that means the dollar signs and the names of the variables, as those are the content of the variable.

$altTitle = 'alt="[$setNumber]:[$questionNumber]"'

The confusing part to me is why you are putting off the variable substitution until the TEXT or PGML step rather than doing it when $altTitle is created originally (the most natural location for the substitution). Because you are using the single-quote form of a string literal, that prevents the variable substitution from occurring. So perhaps

$altTitle = "alt=\"$setNumber:$problemNumber\"";

$altTitle = qq/alt="$setNumber:$problemNumber"/;

Note that PGML's double-star approach allows you to do variable substitution after command substitution, or command substitution after variable substitution (or even variable substitution after variable substitution, or command substitution after command substitution), so it is more flexible than TEXT's substitutions in that sense, but you have to use nested PGML commands, not TEXT commands. E.g.,

$x = 3;
$varsub = 'variable [$x]';
$cmdsub = 'command [@ $x+1 @]';

BEGIN_PGML
[$varsub]**, [$cmdsub]**

[@ $varsub @]**, [@ $cmdsub @]**
END_PGML

variable 3, command 4

variable 3, command 4

So the upshot is that PGML is doing exactly what it is supposed to, and that you probably want to do the variable substitution when you first create $altTitle, not during the PGML processing.