(mysql.info) application-password-use

 
 5.8.9.1 Implications of Password Hashing Changes for Application Programs
 .........................................................................
 
 An upgrade to MySQL version 4.1 or later can cause compatibility issues
 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 or later from a pre-4.1 version of MySQL and run
 the server under conditions where it generates long password hashes, an
 application using `PASSWORD()' for its own passwords breaks.  The
 recommended course of action in such cases 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 for generate short hashes in the old
 format. However, you should 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  php.
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