As usual, there are a couple of different way to accomplish this task.

First, let's discuss the use of Apache. Compared with passing the request to the backend language (in your case PHP), this is usually the preferred way. It's more lightweight and more fast because the webserver doesn't need to push additional levels to the execution stack.

You can write the directive in your Apache configuration file or in the .htaccess file. If you have access to Apache config file, this is the best place. Otherwise, the .htaccess is fine too.

The following directive redirects all request to any non-www host to the www-version of the site.

RewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.domain\.com [NC]
RewriteRule ^(.*)$ http://www.domain.com$1 [L,R=301]

I usually prefer the code above rather the following one that instead selective.y redirect a single domain to the www-version. (note the difference on the second line)

RewriteEngine on
RewriteCond %{HTTP_HOST} ^domain\.com [NC]
RewriteRule ^(.*)$ http://www.domain.com$1 [L,R=301]

Which one you should use depends on your specific case. If the former works, stick with it. Otherwise, if redirecting all host can cause some problem, use the latter.

If you want to use PHP, you can use the following code.

$host = strtolower($_SERVER["HTTP_HOST"]);
if ($host != "www.domain.com") { // or $host == "domain.com"
  $uri = $_SERVER["REQUEST_URI"];
  header("HTTP/1.1 301 Moved Permanently");
  header("Location: http://www.domain.com$uri");
  exit();
}