OK, I got this working, but I needed to do these 2 things:
1) Add the literal space instead instead of the html entity
2) change the OR Separator from a space to the pipe symbol:
$WADbSearch1->keywordComparison($KeyArr1,"".((isset($_POST["descriptionSearch"]))?" ".$_POST["descriptionSearch"]." ":"") ."","AND","Includes",",%20","|%20","%22","%22",0);