Turns out that the problem is not with the double quotes or the single quotes!
I've simplified the query to a simple:
select sectionid from LabelData.dbo.[Section] where sectionid < 41
This works fine, but if I try:
select sectionxml from LabelData.dbo.[Section] where sectionid < 41
where sectionxml is an xml field, it dies with an "unknown error."
Likewise, if I try:
select sectionxmltext from LabelData.dbo.[Section] where sectionid < 41
where sectionxmltext is an nvarchar field, it, too, dies with an "unknown error."
I've loaded the new php_sqlsrv.dll extension recommended by Microsoft, but I'm still getting this error.
So evidently, the problem is with PHP not being able to handle xml or nvarchar field data.
Any suggestions?