title: Most efficient way to change the hash type of a password (md5 to sha1)
body: <p>I have a system using MD5 to hash passwords from my users and store it into my database.
  Now, I'm changing to another system that uses SHA1 (and a unique system SALT, not user-unique) to hash the passwords.</p>
  
  <p>How do I manage to get the user old MD5 password turned into my new SHA1 password with PHP?</p>