Keeping data entered by user in textbox even after clicking submit button

Usually the program enters the month number, for example: 12, then if you press the "Submit" button, it will indicate the name of the month, in this case December.

The problem is that every time I click the "Submit value in textbox" button, I want the textbox to still contain "12" even after I clicked the submit button.

Ps: Ignore non-English words.

    <form method = "post">
4. Insert month(1-12) : 
<input type="text" name="monthTxt" value=""><br/>
<input type="submit" value="Submit">

</form>




<?php
    $monthTemp = $_POST["monthTxt"];
    $testing = $monthTemp;

    function bulan($bulan)
    {
        $months = array(1 => 'Januari', 2 => 'Februari', 3 => 'Maret', 4 => 'April', 5 => 'Mei', 6 => 'Juni', 7 => 'Juli', 8 => 'Agustus', 9 => 'September', 10 => 'Oktober', 11 => 'November', 12 => 'December');

        if($bulan < 1 || $bulan > 12)
        {
            echo "Input tidak boleh kurang dari 0, lebih dari 12, atau huruf";
        }
        else
        {
            echo $months[$bulan];
        }
    }
    bulan($monthTemp);
?>