How do you Encrypt and Decrypt a PHP String?

What I mean is: Original String + Salt or Key --> Encrypted String Encrypted String + Salt or Key --> Decrypted (Original String) Maybe something like: "hello world!" + "ABCD1234" --> Encrypt --> "2a2ffa8f13220befbe30819047e23b2c" (may be, for e.g) "2a2ffa8f13220befbe30819047e23b2c" --> Decrypt with "ABCD1234" --> "hello world!" - In PHP, how can you do this? Attempted to use **`Crypt_Blowfish`**, but it didn't work for me.