SQL stands for Structured Query Language, it is more of a standard than a language. The Database packages which implement SQL are Oracle, MySQL, MS SQL Server 2005, etc. to name a few.
PHP is the server side scripting language used to impart dynamic behaviour to the web pages. And it would be a definate plus on your side if you knew the basics of SQL before attempting to tacke server side scripting languges like PHP since all the dynamic pages do is query the database in one form or the other.
It would be a different thing is you are learing PHP to develop standalone applications using the PHP GTK+. If that is the case then you dont need to know SQL.