Test the conditional attribute value + the "value" attribute in opt:attribute.

>>>>templates/test.tpl
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<opt:root>
<foo>
<opt:attribute name="class" str:value="c">
	<opt:value test="$val1 == 'foo'">a</opt:value>
	<opt:value test="$val1 == 'bar'">b</opt:value>
</opt:attribute>
</foo>
<foo>
<opt:attribute name="class" str:value="c">
	<opt:value test="$val2 == 'foo'">a</opt:value>
	<opt:value test="$val2 == 'bar'">b</opt:value>
</opt:attribute>
</foo>
<foo>
<opt:attribute name="class" str:value="c">
	<opt:value test="$val3 == 'foo'">a</opt:value>
	<opt:value test="$val3 == 'bar'">b</opt:value>
</opt:attribute>
</foo>
</opt:root>

>>>>data.php
$view->val1 = 'foo';
$view->val2 = 'bar';
$view->val3 = 'joe';

>>>>expected.txt
OUTPUT

>>>>result.txt
<foo class="a">
</foo>
<foo class="b">
</foo>
<foo class="c">
</foo>