Elseif statement being ignored - ?

I am new to php and trying a simple load script. I have scaled it down to try to find the problem. When it gets to the elseif statement, it just displays the form. I know that the op and the names are set (I can echo them back just prior to the elseif). If I skip the elseif, the data loads just fine. Any help on the basic, yet frustrating issue would be greatly appreciated.

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<?php
if ($POST[op] != "add") {
$display_block = "<form method=\"post\" action=\"$_SERVER[PHP_SELF]\">

<p>Primary Parent bringing child(ren) to park: <br>
First name: <input type=\"text\" name=\"mem_first\" size=25><br>
Last name: <input type=\"text\" name=\"mem_last\" size=25><br></p>

<input type=\"hidden\" name=\"op\" value=\"add\">

<p><input type=\"submit\" name=\"submit\" value=\"Add Entry\"></p>
</form>";


//These lines echo back the op and names with no problem, just above the form re-display, so I know they are there
echo "$_POST[op];
echo "$_POST[mem_first]";
echo "$_POST[mem_last]";

} elseif ($_POST[op] == "add") {
//time to add to tables, so check for required fields
if (($_POST[mem_first] == "") || ($_POST[mem_last] == "")) {
header("Location: addtest.php");
exit;
}

// I never get this far, unless I take out the elseif.
echo "made it past the else statement";

$conn = mysql_connect("localhost", "root") or die(mysql_error());
mysql_select_db('cip',$conn) or die(mysql_error());


$add_member="insert into member values ('','$_POST[mem_first]', '$_POST[mem_last]', 'new', 'circle',
#'$_POST[status]', '$_POST[park]',
'$_POST[spouse_first]', '$_POST[spouse_last]', 'n',
#'$_POST[daycare_provider]',
now(), now())";
mysql_query($add_member) or die(mysql_error());
.
.
.
.
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Problem solved - I'm pretty embarrassed - total newbie mistake. Forgot underscore in second line. I looked at it all night last night - guess I needed some sleep.