form values not passing through action url in php

Question

I have a simple form made in PHP, I am trying to pass the select value in the page mentioned in the action URL,

<form action="filtertable.php" method="post">
    <select class="form-select" name="filtercategory" aria-label="Default select example"
        onchange="this.form.submit()">
        <option selected>Select Category</option>
        <option value="<?php echo $roww["c_id"]; ?>"><?php echo $roww["cname"]; ?></option>
    </select>
</form>

when the user selects on any select box it will go to the page filtertable.php, and from there am trying to get the selected value like this:

<?php

$filtercategory=$_POST['filtercategory'];

However it doesn't work, can anyone please tell me what is wrong in here, thanks in advance