When you include a file, it is as if the source code in that file were inserted into the calling script's source code at the point where it is included. There is no way to change how that included code is processed other than actually changing the code in the include file.
One approach might be to modify the include file so that the different portions are defined as functions. Then you can include it wherever you want in the main script, then call each function as needed.
include.php:
<?php
function a()
{
echo "This is function a().";
}
function b()
{
echo "This is function b().";
}
main.php:
<?php
require_once 'include.php';
// do some stuff, then...
a();
// do some more stuff, then...
b();
?>