datepicker problem on passing value when submit for second time - php

Question

i have an issue on passing value thru datepicker. i have an appointment system which i use datepicker. i need to get the date and pass the value (on the same page) to my query and check it with my mysql database whether the date available or not. First pick date is ok and it send the right value, but when i try pick another date and submit it, it passing the first date that i choose. Is anyone know what is the problem?
<?php
if (isset($_POST['check']))
{
$appdate = $_POST['appdate'];
}
?>
<form method="post" enctype="multipart/form-data" id="form" name="form">
<div class="form-group row">
<div class="col-md-6">
<label for="jenis">Date Appointment</label>
<input name="appdate" readonly="readonly" id= "appdate" type="text" class="form-control datepicker" data-format="dd-mm-yyyy" data-lang="en" data-RTL="false">
<span class="help-block" id="error" style="color:red"></span>
</div>
<div class="col-md-6">
<label for="jenis"> <br></label>
<input type="submit" name="check" id="check" value="Check" class="btn btn-3d btn-pink"/>
</div>
</div>
<?php
if (isset($_POST['check']))
{
$appdate = $_POST['appdate'];
?>
<div>
<p><?php echo $appdate; ?></p>
</div>
<?php
}
?>
</form>