Posted some time ago (Bug #16814) for Apache 2.0.35/PHP 4.2.0, but still seems to be unresolved.

Using Apache 2.0.39 with php4apache2.dll (PHP 4.2.2) under Win NT 4 SP6, the PHP code won't be executed if the index.php is used as the directory index file.

If http://servername/ is requsted, the PHP code won't be executed, so the plain source shows up as text, which may contain database passwords etc.
With index.php appended to the URL, the file is parsed correctly by the PHP module.

From my Apache httpd.conf:

LoadModule php4_module C:/php-4.2.2-win32/sapi/php4apache2.dll
AddType application/x-httpd-php .php .php3 .php4 .phtml
AddType application/x-httpd-php-source .phps
AddOutputFilter PHP;INCLUDES .php .php3 .php4 .phtml .phps
[...]
DirectoryIndex index.php index.php3 index.php4 index.phtml index.html index.html.var index.htm

As a workaround, I use an index.html with a <META> redirect to index.php.