It is easy to include another file in your PHP file.
The function is include '';
Here is how you use it. For example, you have developed a website template, and you break down the HTML file into header, content, and footer, and you have header.htm, content.htm and footer.htm, respectively.
Say you file is index.php, you would simple include your template files like this:
<?
include 'header.htm';
include 'content.htm';
include 'footer.htm';
?>