Normally you would just do an include or require:
<?php
<?php
$niche = $_REQUEST['sub-niche'];
if ( $niche == "golden_retrievers" ) {
include "golden_retrievers.php";
}
Depending on the exact logic/functionality involved, you might do a redirect instead via the header() function, but that adds some internet communications overhead so should only be done if an include will not suffice for some reason.