What is more efficient and/or what is better practice, to echo the HTML or have many open and close php tags?
Obviously for big areas of HTML it is sensible to open and close the php tags. What about when dealing with something like generating XML? Should you open and close the php tags with a single echo for each piece of data or use a single echo with the XML tags included in quotations?
From a maintenance perspective, one should have the HTML / XML as separate from the code as possible IMO, so that minor changes can be made easily even by a non-technical person.
The more a homogeneous block the markup is, the cleaner the work.
One way to achieve this is to prepare as much as possible in variables, and using the heredoc syntax:
// Preparation
$var1 = get_value("yxyz");
$var2 = get_url ("abc");
$var3 = ($count = 0 ? "Count is zero" : "Count is not zero");
$var4 = htmlentities(get_value("def"));
// Output
echo <<<EOT
<fieldset title="$var4">
<ul class="$var1">
<li>
$var2
</li>
</ul>
</fieldset>
EOT;
You will want to use more sensible variable names, of course.
Edit: The link pointed out by @stesch in the comments provides some good arguments towards using a serializer when producing XML, and by extension, even HTML, instead of printing it out as shown above. I don't think a serializer is necessary in every situation, especially from a maintenance standpoint where templates are so much more easy to edit, but the link is well worth a read. HOWTO Avoid Being Called a Bozo When Producing XML
Another big advantage of the separation between logic and content is that if transition to a templating engine, or the introduction of caching becomes necessary one day, it's almost painless to implement because logic and code are already separated.