I am primarily a designer with novice level php experience, so go easy on me 
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I am moving an existing site from an asp .NET server to one for which I will rebuild the site entirely using php. My first challenge is pulling various bits of content into pages using 'include' ( <?php include 'includes/some_content.php'; ?> ). The hurdle here is trying to understand how I can do this via a list of links, such as a side bar which pulls in a different bit of content depending on which link is clicked. So the list of links is in the parent/master page with a div for the content.
How can I:
1. Set a default content include for this master/parent page.
2. Have that default content include replaced when sidebar links are clicked?
I have tried to search for info on how this can be done, but am having difficulty locating it despite it being a seemingly basic concept. Example attached.
