php validation removes the data from html input - php

Question

I'm having some problem with html input validation using PHP. The validation itself is working. I have two inputs: name and office.
If I enter a value on Name input but i didn't put value to the office input and click the submit button, the validation on office input works but it CLEAR/null the data i entered on name input
What am I doing wrong here?
This is my PHP validation:
if (isset($_POST['submit'])){
$signatory_name = $_POST['sig_name'];
$signtory_position = $_POST['sig_position'];
if (!$_POST['sig_name']) {
$errname='<div class="alert alert-danger">Sorry there was an error: Please Enter Your Name</div>';
}
if (!$_POST['sig_office']) {
$erroffice='<div class="alert alert-danger">Sorry there was an error: Please Enter Your office</div>';
}
}
this is my html code
<form action="signatory.php" method="Post" role="form">
<input class="form-control " id="signatoryname" name="sig_name" placeholder="Name:" >
<input class="form-control " id="signatoryoffice" name="sig_office" placeholder="Office:">
</form>

Answer 1

It's not really clear what you are doing or trying to do but here is my attempt:
First: You should know that if (!$_POST['sig_name']) { means if the value assigned is FALSE you may want to reconsider this and use empty() instead.
After validating the inputs you need to repopulate the form with the submitted values - here is an example:
<?php
$errname = "";
$erroffice= "";
if (!empty($_POST)) { // Only if there are POST values attached.
$signatory_name = $_POST['sig_name'];
$signtory_position = $_POST['sig_position'];
if (empty($_POST['sig_name'])) {
$errname='<div class="alert alert-danger">Sorry there was an error: Please Enter youre Name</div>';
}
if (empty($_POST['sig_office'])) {
$erroffice='<div class="alert alert-danger">Sorry there was an error: Please Enter youre office</div>';
}
if (empty($errname) && empty($erroffice)) {
//Do whatever you need with the validated inputs...
} else {
//Expose the alerts:
echo $errname.$erroffice;
}
}
?>
<form method="POST" role="form">
<input class="form-control" id="signatoryname" name="sig_name" value="<?php echo (isset($_POST['sig_name']))?$_POST['sig_name']:""; ?>" placeholder="Name:" />
<input class="form-control" id="signatoryoffice" name="sig_office" value="<?php echo (isset($_POST['sig_office']))?$_POST['sig_office']:""; ?>" placeholder="Office:" />
<!-- rest of your form and buttons -->
</form>

Answer 2

You need to re-populate your form as Scuzzy said.
Most browsers might do you for you, but you can't rely on it.
<form action="signatory.php" method="Post" role="form">
<input class="form-control " id="signatoryname" name="sig_name" placeholder="Name:" value="<?php echo !empty($_POST['sig_name']) ? $_POST['sig_name'] : ''; ?>">
<input class="form-control " id="signatoryoffice" name="sig_office" placeholder="Office:" value="<?php echo !empty($_POST['sig_office']) ? $_POST['sig_office'] : ''; ?>">
</form>