You can't, because crypt()
isn't an encryption function, it's a one-way hash function.
Note: There is no decrypt function, since crypt() uses a one-way algorithm.
You shouldn't do what you're describing anyway. If an attacker has access to someone's email and every website sent the "lost password" in plaintext to the user upon request, that attacker could gain even more passwords.
You should let the user enter a new password upon proving their identity (maybe by answering a question) and possession of that email.