JSP equivalent to the PHP include() function?

You have a couple of options. The first is <jsp:include>. The second is <c:import>. The c: tags are JSTL, the JavaServer Pages Standard Tag Library.

What's the difference? Primarily <jsp:include> inserts the contents of another JSP page within the same JAR relative to the current page whereas <c:import> can read in an absolute or relative URL and display those contents on the page, retrieve a Reader or store the contents in a variable.

The syntax for both is XML-like so:

<jsp:include page="header.jsp"/>

or

<jsp:include page="header.jsp"></jsp:include>

Note: both can take parameters.