Is it good practice to add a php include of the head section in my pages?
Yep, it's quite standard. But instead of writing:
<head>
<?php include('head.php'); ?>
</head>
you should put the tags inside head.php. I say it's better because what's inside head.php has no sense without the head tags, so they are kinda linked together. It's good practice to join things so linked into a single file without having to repeat open and close head tags for each page.
Actually, it's even good practice (and commonly used) to have header.php, body.php and footer.php files that has respectively:
header.php
<html>
<head>
...
</head>
<body>
body.php
...
footer.php
</body>
</html>
I'm doing that in my application but I've found that it's not a good idea, because you have many of your stylesheets, javascripts, etc in a php file including the head section and you'll have problems with including it in php files in nested folders. this problem is because of relative paths. If you can use absolute paths then it's ok otherwise it's not a good idea ...
PHP Includes are used like this all the time. Any time that you have content that will be the exact same on every page, it is very helpful to use an include