I'm new to PHP as well and can't answer this, but I can offer an alternative.

What if you edited the email language file for the welcome email to tell them that their user name/logon is the email address that they received their welcome email to?

If you do that, you might also mention that for security purposes, you are unable to provide the password and remind them to keep it in a secure location, and perhaps tell them about the password reset function in case they forget it.