I wanted to test a script written for PHP 5.3.5 (installed as Apache module) in PHP 5.2 environment.
I installed a fresh copy of Apache 2.0.64 and PHP 5.2.17 and configured it EXACTLY as I had configured PHP 5.3 before. Except I did copy libmysql.dll into Apache bin directory (PHP 5.3 doesn't have this file anymore).
Both servers are configured by hand (not a XAMPP versions or something) and access the same locally installed mySQL Server 5.5.8. Both servers are running as Windows Server under the system account.
PHP 5.3 works perfectly, PHP 5.2 returns an error:
Warning: mysql_connect() [function.mysql-connect]: Access denied
for user 'SYSTEM'@'localhost' (using password: NO) in C:\Tools\htdocs\myscript.php on line 33
The SQL-user I am using is hardcoded in PHP code ant it is not SYSTEM@localhost. SYSTEM seems to be a Windows account the Apache is running under. (If I start the Apache from the Administrator account, the message changes to Administrator@localhost.)
The default user (with a name '') was deleted on my SQL-Server. If this empty-user exists, the PHP 5.2 connects to the database successfully, but fails on the database selection (mysql_select_db) because of permissions.
If the sql.safe_mode (NOT the PHP safe mode) in on in the php.ini, PHP is always using the script owner to connect to the SQL server and not the user specified in mysql_connect().
I mixed up the property safe_mode and sql.safe_mode during the configuration and it cost me 6 hours!
The answer was found here: http://www.php.net/manual/de/ref.mysql.php#72632