From Apache FAQ:
Why do I keep getting "Method Not Allowed" for form POST requests?
This is almost always due to Apache not being configured to treat the file you are trying to POST to as a CGI script. You can not POST to a normal HTML file; the operation has no meaning. See the FAQ entry on CGIs outside ScriptAliased directories for details on how to configure Apache to treat the file in question as a CGI.
Your apache does not handle .php (.php3) files as CGI or ExecCGI is not allowed in particular directory - check Options.