How to persist the selected value of the select box after form submit?

 <?php
if ($_POST['submit']) {
    if ($_POST['srf'] != "") {
        $srf = $_POST['srf'];
    } else {
        $srfErr = "This field is required.";
    }
}
?>
<form method="post">
<select name="srf" id="srf" class="form-control">
<option value="">Select</option>
<option <?php if (isset($srf) && $srf=="1") echo "selected";?>>1</option>
<option <?php if (isset($srf) && $srf=="2") echo "selected";?>>2</option>
<option <?php if (isset($srf) && $srf=="3") echo "selected";?>>3</option>
</select> <?php if($srfErr!= ""){ ?>
        <p><b><?php echo $srfErr;  ?></b></p>
</div>
    <?php }?>
<input type="submit" name="submit" value="submit" />
</form>