- array_slice: documentation ( source)
- array_merge: documentation ( source)
- implode: documentation ( source)
- explode: documentation ( source)
<?php
$current_url = 'current.sub.meh.com';
$stage_url = 'foo.stage.it';
$current_domains = explode( '.', $current_url );
$current_sub_domain = array_slice( $current_domains, 0, count( $current_domains ) - 2 );
$stage_top_domain = array_slice( explode( '.', $stage_url ), - 2, 2 );
$stage_url = implode( '.', array_merge( $current_sub_domain, $stage_top_domain ) );
echo $stage_url;