While the PHP developers have done their utmost to ensure compatibility, we cannot guarantee that PHP 4 scripts will function 100% correctly under PHP 5. Please note that in contrast to PHP 4, the default for the variable RegisterGlobals is "off". By default Apache uses PHP 4 for .php extension. If you don't want to rename all your scripts to .php5 you can do the following: Create a .htaccess file and place the following line AddType x-mapp-php5 .php in it. This will tell Apache to use PHP 5 instead of PHP 4 for the extension .php in the directory the .htaccess is placed and all sub-directories under it.