Simplest way to stop email submission if field is empty - php

Question

Im trying to duplicate my client side validation on the server (PHP). What is the simplest way to make sure a field has been filled out? I've done it like this before but am having some issues, is there a better way?
<?php
if(empty($_POST['Contact0FirstName']) || empty($_POST['Contact0Email']) ||
empty($_POST['Contact0Phone1']) || empty($_POST['CaptchaInput']) || !empty($_POST['LeadBlind'])) {
echo "Error";
die();
} else {
//set POST variables
$url = 'xxx';
$infusion_xid = $_POST["xxx"];
$infusion_type = $_POST["xxx"];
$infusion_name = $_POST["xxx"];
$Contact0FirstName = $_POST["Contact0FirstName"];
$Contact0Email = $_POST["Contact0Email"];
$Contact0Phone1 = $_POST["Contact0Phone1"];
$fields = array(
'infusion_xid'=>urlencode($infusion_xid),
'infusion_type'=>urlencode($infusion_type),
'infusion_name'=>urlencode($infusion_name),
'Contact0FirstName'=>urlencode($Contact0FirstName),
'Contact0Email'=>urlencode($Contact0Email),
'Contact0Phone1'=>urlencode($Contact0Phone1)
);
//url-ify the data for the POST
foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
$fields_string = rtrim($fields_string,'& ');
//open connection
$ch = curl_init($url);
//set the url, number of POST vars, POST data
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields_string);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
//their return html
$output = curl_exec($ch);
//echo "TEST";
echo $output;
curl_close($ch);
}
?>
<form id="Lead" method="post" action="PHP/Lead.php" accept-charset="UTF-8">
<ul id="LeadForm">
<li>*required field</li>
<li>
<input type="hidden" name="LeadBlind" id="LeadBlind">
<label for="Contact0FirstName">1 | First Name*</label>
<label for="Contact0Email">2 | Email*</label>
<label for="Contact0Phone1">3 | Daytime Phone*</label>
</li>
<li>
<input type="text" name="Contact0FirstName" id="Contact0FirstName">
<input type="text" name="Contact0Email" id="Contact0Email">
<input type="text" name="Contact0Phone1" id="Contact0Phone1">
</li>
<li>
<label for="CaptchaInput">4 | Please enter the code</label>
</li>
<li>
<input type="text" class="numbers" name="Captcha" id="Captcha" value="" readonly>
<input type="text" class="numbers" name="CaptchaInput" id="CaptchaInput" size="6" maxlength="6">
</li>
<li>
<input type="submit" id="LeadSend" value="Try It Now!">
<span id="processing">Submitting Your Request</span>
</li>
</ul>
<div class="clear"></div>
</form>
I need "LeadBlind" to be empty, it is a hidden input that only form filling bots would fill in, so if it has a value it should trigger the error. The ajax function is based on what is returned, if it's "error" it displays an error msg, otherwise a success one. thx for the help!