A bit more info...

Variables (and expressions) in KiXtart are very flexible when it comes to typing. Basically the left hand element in any expression coerces the expression type. Be careful about operator precedence though.
For instance,
code:
$i="A" + 5 * "3"
Will work, and the result will be "A15"
code:
$i="A" + 5 + "3"
Will work, and the result will the "A53"

In the first example, the '*' operator has the highest precedence so '5 * "3"' is evaluated first. Because the '5' on the lefthand side of the equation is numeric the string '"3"' is converted to a numeric and the multiplication succeeds with the result '15'. Now the expression becomes '"A" + 15'. The left hand side of the expression is now a string, so the '15' is converted to a string. This causes the '+' to operate as a catenation operator, so the result is 'A15'.

In the second example the two '+' have the same precedence, so are evaluated left-to-right.
The string '"A"' causes the '5' to be converted to a string, so the operation is a catenation resulting in the string '"A5"'. The next expression becomes '"A5" + "3"', which again is a string expression, so the result is another catenation with the result '"A53"'