Selecting every 3rd item starting after the 2nd - php

Question

I have the following code:
$loop = 0;
$columns = 3;
<?php while ( have_posts() ) :
the_post();
$loop++;
?>
<li class="<?php if ( $loop % $columns == 0) echo 'last'; if ( ( $loop - 1 ) % $columns == 0 ) echo 'first'; ?>">
</li>
What I want to achieve is:
<li class="first"></li>
<li class="last"></li>
<li class="first"></li>
<li class=""></li>
<li class="last"></li>
<li class="first"></li>
<li class=""></li>
<li class="last"></li>
What I am getting is (which makes sense..):
<li class="first"></li>
<li class="=""></li>
<li class="last"></li>
<li class="first"></li>
<li class=""></li>
<li class="last"></li>
<li class="first"></li>
<li class=""></li>
<li class="last"></li>
meaning the first 2 are given the class first and last respectively and after that it is every 3.
If i was able to use css I would say nth-child(3n+2) but I can't because I want to assign a new class, and I can only assign a style with css.
How can I achieve it with php?

Answer 1

The following will generate the code that you gave as an example:
<?php
$loop = 0;
$offset = 2;
while ($loop < 100) {
echo '<li class="';
if ($loop < $offset) {
if ($loop % 2 == 0) {
echo 'first';
} else {
echo 'last';
}
} else {
if (($loop - $offset) % 3 == 0) {
echo 'first';
} elseif (($loop - $offset + 2 ) % 3 == 0) {
echo '';
} else {
echo 'last';
}
}
echo '"></li>';
++$loop;
}
If the $loop variable is under the offset it print first and last alternately otherwise it will check divisibility with 3, giving the dividend an extra offset for each condition so they will be true sequentially.