I need to capture the value of the option deadlinemaxsale that the user selects in the select charge_field and use it in the sql query resultsplpag. This way display the related results in select plpag_field. However, when the user is changing the option, the $prazomaxvenda variable is not being updated. Here's the code for the function I'm creating.

function custom_checkout_field( $checkout ) {
    $current_user = wp_get_current_user(); 
    global $wpdb;
    
    $results = $wpdb->get_results("SELECT codcob, cobranca, prazomaxvenda FROM wptkyb_cobranca WHERE nivelvenda >= (SELECT nivelcli FROM wptkyb_nivelclientes WHERE cgcent =  $current_user->user_login)");
    echo '<div id="custom_checkout_field" how="formas-pagamento"><h3>'.__('Formas de pagamento:').'</h3>';
    echo '<select name="cobranca_field" id="cobranca_field>';

    foreach ($results as $result) {
        echo '<option value="' . $result->codcob . '">' . $result->cobranca . '</option>';
    }
    
    $prazomaxvenda = $results[0]->prazomaxvenda; // armazena o valor de prazomaxvenda
    
    echo '</select></div>';//Fim da div de cobranca_field
    echo "Valor de prazomaxvenda: $prazomaxvenda"; 
    echo '<div id="custom_checkout_field2" how="prazo-pagamento"><h3>'.__('Prazo de pagamento:').'</h3>';
    echo '<select name="plpag_field" id="plpag_field">'; 
 
    $resultsplpag = $wpdb->get_results("SELECT codplpag, descricao FROM wptkyb_plpag WHERE prazomedio <= $prazomaxvenda");

    foreach ($resultsplpag as $result) {
        echo '<option value="' . $result->codplpag . '">' . $result->descricao . '</option>';
    }

    echo '</select></div><br>';
}// End function

I tried to change the position of the variable but it didn't work, I think I have to capture the change event in the select but I couldn't do it. If the variable changes according to the user's selection, I think it will work fine