My lease states that "Keys are for TENANT'S use only. Keys are not to be given to anyone who is not listed in the lease."

I would like to give a copy of my apartment key as well as the key to the apartment complex itself to my girlfriend, who is not on the lease and does not live with me. My landlords forbid it and they're probably within their rights, but I would just like to double check if this is legal in Philadelphia.

Thanks

Is it legal to hold you to a contract you signed?

Absolutely.

And they can decide that violating that provision of your contract makes the entire contract null and void.