How to move featured image to the top of the list?

Question

Can’t you do this in HTML? So instead of printing everything in your loop, append it to a string that you print in the end. That way you can keep special stuff apart.

I assume this is a follow-up question to your previous question about featured images, so I will use the code from my answer there as an example.

<?php
$testimonials = get_posts( array(
    'category_name' => 'testimonial',
    'numberposts' => -1,
    'order' => 'DESC'
) );
$highlighted_testimonial_thumbnail="";
$other_testimonial_thumbnails="";
foreach ( $testimonials as $testimonial ) {
    if ( $testimonial->ID == $current_testimonial_id ) {
        $highlighted_testimonial_thumbnail="<li class="highlighted">" . get_the_post_thumbnail( $testimonial->ID, 'nav' ) . '</li>';
    } else {
        $other_testimonial_thumbnails .= '<li>' . get_the_post_thumbnail( $testimonial->ID, 'nav' ) . '</li>';
    }
}
echo '<ul class="portfolio">';
echo $highlighted_testimonial_thumbnail;
echo $other_testimonial_thumbnails;
echo '</ul>';

Or more generic: instead of:

foreach ( $array_of_stuff as $stuff ) {
    echo $stuff
}

Do it like this:

$output="";
$featured_output="";
foreach ( $array_of_stuff as $stuff ) {
    if ( is_featured( $stuff ) ) {
        $featured_output = $stuff;
    } else {
        $output .= $stuff;
    }
}

echo $featured_output;
echo $output;

Almost all functions in WordPress that echo something have an equivalent that doesn’t echo but just returns it. get_the_content() vs the_content() for example.