Smart quotes are more trouble than they're worth. To me, they should be banned forever from any publication.
Unfortunatelly, the PHP certification guide is using smart quotes even in the code, I hope they are not confusing the people that are just getting started with PHP. The most confusing, I think, is the example where the text explains how quotes can be escaped in strings.
So, to all readers of the PHP cetrtification giude:
Don't pay attention to the direction of the smart quotes!
‘
’
and
'
are all born equal.
Same goes for
“
”
and
"
