//Here the user sets which table fields will be used:
<input type="checkbox" name="id" value="1"> ID
<input type="checkbox" name="name" value="1"> name
<input type="checkbox" name="email" value="1"> email

//Here I see what fields the user wants to use and set a variable to 1

<?php

if (isset($_POST['id']) && $_POST['id'] == "1") {

    $form_id='1';
}

//etc etc

$sql = mysql_query("select * from mytable");

$sql = mysql_query("select id, name from $table");

$fields = [];
if(isset($_POST['id']) && $_POST['id'] == 1) {
    $fields[] = 'id';
}

$fields = implode(', ', $fields);