php $_POST var clearing itself when i check it - php

Question

i have a crazy problem that i just can't figure out. my form has two fields and a submit button. when i submit the vars get passed into $_POST just fine...
print('<div class=error>');
print_r($_POST);
print('</div>');
that gives me the two fields with the expected values along with the value of the submit button.
HOWEVER! when i add the following line of code so i can process based on the submit button, it clears all of the data. the post array shows up empty.
if ($_POST['submit'] == 'Submit') {
that clears the data. if i change the value from 'Submit' to anything else, the vars still show up in $_POST, they just get cleared when i try to check them.
any ideas what i'm doing wrong here?
here's the form:
<form method="post" action="<?php echo $_SERVER['PHP_SELF'];?>">
<?
if (isset($msg)) {
echo "$msg";
}
?>
<input type=text name='email'><br>
<br><input type=password name='password'>
<br>
<input type="submit" name="submit" value=Submit>
</form>
and here's the processing code:
if ($_POST['submit'] == 'Submit') {
echo "<div class=error>made it here</div>";
$u = $_POST['email'];
$p = $_POST['password'];
$auth = mysql_query("Select * from member where email='$u' and password='$p'");
$auth = mysql_fetch_array($auth);
if ($auth) {
$pid = $auth[id];
echo "aa";
sess_register("sess_msg");
$sess_msg = null;
global $auth, $pid;
}
}
if i change the value when i check to see if the submit button has a value to something other than the actual value of the submit button, which is 'Submit' - it clears all variables sent to $_POST

Answer 1

If you want to check which submit button was clicked, you just have to look for its name as a key in the array $_POST.
So you should do:
if (array_key_exists('submit', $_POST)) {
// your code
}
Little advice: you'd better escape your $_POST data before putting it into a query!
Check this out: http://php.net/manual/en/function.mysql-real-escape-string.php

Answer 2

From your post, it doesn't look your code should be emptying the $_POST array. The only thing that I can think of at the moment is that maybe in the code you actually only put one '=' sign.
var_dump( $_POST );
if ( isset( $_POST['submit'] ) ) {
var_dump( $_POST );
echo "<div class=error>made it here</div>";
$u = $_POST['email'];
$p = $_POST['password'];
$auth = mysql_query("Select * from member where email='$u' and password='$p'");
$auth = mysql_fetch_array($auth);
if ($auth) {
$pid = $auth[id];
echo "aa";
sess_register("sess_msg");
$sess_msg = null;
global $auth, $pid;
}
}
var_dump( $_POST );