Amen.... I've started to use more than one language, and it's made me a better programmer and far more understanding of how different languages approach problems.
PHP does have problems, but it's easy to pick up. I don't know that learning it will make you a better programmer, but a good programmer can definitely make it solve problems efficiently enough.
Any new language you learn will make you a better programmer.
Try picking one that is as far as possible from the ones you know and then try building something useful in it. That's the fastest way to broaden your horizons.
PHP does have problems, but it's easy to pick up. I don't know that learning it will make you a better programmer, but a good programmer can definitely make it solve problems efficiently enough.