[If statement issue...]

I'm having this issue with form validation..
Basically it just skips all my if statements such as:
[code]
if(!$_POST["first_name"] || !$_POST["last_name"] || !$_POST["email"] || !$_POST["mailing_street"] || !$_POST["mailing_city"] || !$_POST["mailing_state"] || !$_POST["mailing_zip"])
    header("location: profile.php?reason=info");
if($password1 != $password2)
    header("location: profile.php?reason=password");
if(check_email_address($emailaddress) == false)
    header("location: profile.php?reason=email");
if($mailingstate == "XX")
    header("location: profile.php?reason=info");
[/code]
And goes right to the database portion where it dutifully updates the person's record with any gibberish they put in.
Interestingly enough, if I put a die("dead"); anywhere in the code, the if statements magically work.
I'm thinking there is some kind of syntax error i'm just not seeing, is there anything I should be looking for? This has had me stumped for awhile.