problem checking value of POST variable

I have a variable which is being posted into a page.

The variable came from a textbox.

All I want to do is check the value of the variable is valid.

code below:
                global $p_forename;
                                    
            if($p_forename ==  "" || "Enter Your First Name")
            {
                  $p_forename = "Enter Your First Name";
                  $invalid = 1;
            }

If I do an echo on $p_forename before the check it contains a value that should jump over this check.....

If the value is invalid a return to the page in which it was entered.

in this example i should be moving on but it keeps going back to the entry page and populating the forename textbox with "Enter Your First Name" which shouldnt be happening.

Can anyway see what im doing wrong.

cheers