PHP echo variable and html together

<?php
    $variable = 'testing';
    echo <span class="label-[variable]">[variable]</span>
?>

Should be

<?php
    $variable = 'testing';
    echo '<span class="label-' . $variable .'">' . $variable . '</span>';
?>

If your HTML contains double quotes, then you can wrap it in single quotes:

echo '<span class="label-' . $variable . '">' . $variable . '</span>';

Additionally, you can do it inline by escaping the double quotes:

echo "<span class=\"label-$variable\">$variable</span>";

Using double quotes also allows for special characters such as \n and \t.

The documentation is your friend.

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