Field disabled does not pass to $ _POST

I have problems with the field that I disable if one of the options is checked, but when the field is as desabled it does not pass values to PHP.

I would like to know how to check whether the value exists or not, and if it does not exist, set it to null.

I tried to do in several ways but the same error happens:

  

PHP Notice: Undefined index: f_DAT_FINAL_VINCU

The attempts were:

   $campos[':DAT_FINAL_VINCU'] = ((isset($_POST['f_DAT_FINAL_VINCU'])) ? eglise_dataDeDMAParaAMD($_POST['f_DAT_FINAL_VINCU']) : null ); 
    $campos[':DAT_FINAL_VINCU'] = strlen(eglise_dataDeDMAParaAMD($_POST['f_DAT_FINAL_VINCU'])) === 0 ? null : htmlspecialchars($_POST['f_TXT_DESCR_VINCU'], ENT_QUOTES); 
    $campos[':DAT_FINAL_VINCU'] = strlen(eglise_dataDeDMAParaAMD($_POST['f_DAT_FINAL_VINCU'])) === null ? null : htmlspecialchars($_POST['f_TXT_DESCR_VINCU'], ENT_QUOTES); 

How do I do it?

I'm putting the field via jQUERY but it's not working looks like I do:

        var variavel = $('#f_FLG_STATU_VINCU').val();
    if (variavel == "A") {
        $('#f_DAT_FINAL_VINCU').val(null);
    }
    $('#f_FLG_STATU_VINCU').change(function () {

        var variavel = $(this).val();
        var today = new Date();

        if (variavel == "A") {
            $('#f_DAT_FINAL_VINCU').val(null);
            $('#f_DAT_FINAL_VINCU').prop("readonly", true);
        } else {
            $('#f_DAT_FINAL_VINCU').prop("readonly", false);
            $('#f_DAT_FINAL_VINCU').datepicker("setDate", today);
        }
    });