I am not sure that storing a plain password like that into the database is a good idea. Maybe you could mitigate the "risk" exposure because of that.
I mean that you have the password "hello", but your custom PHP code hashes it and stores the value "85121215" into the database. This simple bidirectional hash only replaces a given letter by its corresponding number in the alphabet. That's a simple example to illustrate what I mean.
The point is that if a cracker can access this value, he/she won't be able to decipher it, unless he gains access to the PHP logic behind it.
How would she figure out that those numbers are representing letters?