wpdb query with dynamic column name?

Question

This is simply a syntax error by the looks of it. You are using " to open the string, and ' to close it. Use this:

$which_column = 'phone';
$data_check = $wpdb->get_var("SELECT " . $which_column . " FROM wp_shopping_preferences WHERE wp_user_id = '$id'");

Fun fact: when using double quotes ("), you can directed insert a variable into the string without closing it~

$which_column = 'phone';
$data_check = $wpdb->get_var("SELECT $which_column FROM wp_shopping_preferences WHERE wp_user_id = '$id'");

EDIT

I noticed that you have $id at the end of the query also being added. **If this is a user-submitted valueyou must use [$wpdb->prepare`]1 to ensure it’s sanitized properly. See below:

$which_column = 'phone';
$data_check = $wpdb->get_var( 
    $wpdb->prepare( 
        "SELECT $which_column FROM wp_shopping_preferences WHERE wp_user_id = %d",
        $id 
    )
);

The extra line breaks are unnecessary, but only to demenstrate what’s happening. I’ve replaced$id with %d, which will typecast $id as a numeric value. Check out $wpdb->prepare for more options.