An upgrade to MySQL 4.1 can cause a compatibility issue for applications that use PASSWORD() to generate passwords for their own purposes. Applications really should not do this, because PASSWORD() should be used only to manage passwords for MySQL accounts. But some applications use PASSWORD() for their own purposes anyway.

If you upgrade to 4.1 and run the server under conditions where it generates long password hashes, an application that uses PASSWORD() for its own passwords breaks. The recommended course of action is to modify the application to use another function, such as SHA1() or MD5(), to produce hashed values. If that is not possible, you can use the OLD_PASSWORD() function, which is provided to generate short hashes in the old format. But note that OLD_PASSWORD() may one day no longer be supported.

If the server is running under circumstances where it generates short hashes, OLD_PASSWORD() is available but is equivalent to PASSWORD().

PHP programmers migrating their MySQL databases from version 4.0 or lower to version 4.1 or higher should see Section 17.7, “MySQL PHP API”.