If I could only give you one reason why I consistently use single quotes then I would have to go with this:
If you ever (God forbid) need to write HTML in a string, you don’t need to escape the double quotes around attribute values. Instead of:
"<div id=\"foo\" class=\"bar\">foobar</div>"
'<div id="foo" class="bar">foobar</div>'
The other reason is that if you use double quotes, PHP will actually parse the contents of the string to find variables and escape characters (\n, \r, \t etc.) which it’ll replace with the variable value or correct escape character respectively. So for example:
<?php $baz = 'bar'; echo "foo\n$baz"; ?>
<?php $baz = 'bar'; echo 'foo\n$baz'; ?>
So if you don’t need any of this functionality in your string, you’re making PHP do some extra work for no reason.