echo "<span class=\"blue\">$host</span>";

echo "$host";

echo $host;

	$('.doit').click(function() {
		$( "input:checked" ).each(function(){
			$(this).attr('checked',false);
			var $this = $(this);
			$.ajax({
				type: "POST",
				url: "ajax_backlinks.php",
				data: {name: this.name, host: this.value},
				success: function (data) {
					$( data ).insertAfter( $this.parents("label") );
				}
			});
		});
	});

<?php
ini_set('error_reporting', E_ALL);
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);

// работает
echo "<span class=\"blue\">eee</span>";

// НЕ работает
echo "eee";

<table>
  <tbody>
    <tr>
      <td>
        <div><label><input type="checkbox" name="urls" value="url_01" class="checkbox_urls" /></label></div>
      </td>
    </tr>
  </tbody>
</table>
<button type="button" class="doit">Работать!</button></p>

        <div><label><input type="checkbox" name="urls" value="url_01" class="checkbox_urls" /></label>eee</div>