Need help for PHP encode and decode with a security key

Question

I want to encode links for hiding the source. I tried simple PHP base64_encode and I was happy but the problem is that anyone can easily decode that code. So if I can add one security key then it'll be more secure.

encode = "ABCDEFG" + "KEY" = iusdnfiovuccnxcjdkf (output only text and number like base64)
    
decode = "iusdnfiovuccnxcjdkf" + "KEY" = ABCDEFG