Its not actually sites with PHP theres JSP, ASP etc sites that do it as well.
I prefer that I didnt get an email with my Password in it, I feel insecure if that was the case, although it does help remember it (however that is why Lost Password functions were created).
There are a few sites that do send me my password when I sign up some even send it again in Newsletters (well did anyway made sure my account was deleted quick smart).
By all means if you feel its required do so, otherwise you will find some users like myself will not like it, and may leave.