[Q] Why does my php file gets displayed instead of executed

When a user first comes to my site, the user's session id is checked and validated. If no session id is present or the validation fails, the user is vectored to a second page containing a login form. When the user enters the username and password and then clicks the submit button the info is forwarded to a third page, a php script, validate_member_login.php, that checks the username and password against a database. Just for completeness, note that the php script, validate_member_login.php, is invoked via login form's action parameter, i.e., <form action="validate_member_login.php" ... /> The problem is that the php script, validate_member_login.php, is displayed in the browser rather than being executed. This is my first attempt at designing a dynamic web site so I'm sure I've missed something really basic, but I have hardly any hair left to pull out. Thanks, in advance, Michael