I ran into this on another project a while back and the approach that we took was to read the separator from the php.ini file and place it into a constant then use the constant wherever you wanted a separator in a querystring. There are (a lot of) people who care less about standards conformant code. For those that do care, you can set the separator you want on a per script basis (perhaps a semicolon):
<?php ini_set('arg_separator.output',';'); ?>
I’m not familiar with WP enough to know where their constants are set-up, but you can read these php.ini settings using ini_get. Perhaps someone at WP already set up an application constant with this setting in it? Would it make sense to concatenate that in place of an html escaped entity?
Separators note at W3C
