PHP: highlight alternate rows in HTML output

The examples below assume we want to display a list of items on an HTML page and attach a CSS class called "highlight" only to every other item:

<?php
// If we know a collection's row numbers are sequential, we can check for even row numbers using
// the modulo operator % and highlight only even (or odd) rows
$items = array(1 => 'mug', 2 => 'toaster', 3 => 'kettle', 4 => 'plate', 5 => 'spoon');

foreach ($items as $index => $item) {
    echo '<span class="item' . ($index % 2 == 0 ? ' highlight' : '') . '">' . $item . '</span><br />' . PHP_EOL;
}

// When we can't rely on row numbers being sequential, we can create a boolean variable and flip its
// value (by assigning it to a negated version of itself with !) on every iteration of the loop
$items = array(3 => 'mug', 7 => 'toaster', 2 => 'kettle', 8 => 'plate', 9 => 'spoon');
$shouldHighlight = false;

foreach ($items as $item) {
    echo '<span class="item' . ($shouldHighlight ? ' highlight' : '') . '">' . $item . '</span><br />' . PHP_EOL;
    $shouldHighlight = !$shouldHighlight;
}

/*
Both of the examples above print the following:
<span class="item">mug</span><br />
<span class="item highlight">toaster</span><br />
<span class="item">kettle</span><br />
<span class="item highlight">plate</span><br />
<span class="item">spoon</span><br />
*/