PHP is probably the most popular web scripting language used today. PHP is popular because it's easy to learn and use - generally PHP makes life more exciting. Now that I’ve said that, I guess I should actually give you a reason or two to learn PHP!
You will be able to do all kinds of neat things like process HTML forms, send mail from a web page, talk to a database et cetera.
You will become a much more valuable person! Besides your web design skills, you will now have what is called ‘server-side programming’ skills – in this case, knowing PHP.
As depressing as it may seem, programmers usually are higher up on the totem pole. In other words, knowing how to program will put you in a higher bracket than people who only know web design.