[PHP] Variable dans le champ d'une table SQL

<?php
$id = 4;
echo 'c$id'; // affiche c$id
echo "c$id"; // affiche c4
?>

$req = "INSERT INTO ips(c{$id}) VALUES(...)";

echo '<pre>'; 
print_r($bdd->errorInfo()); 
echo '</pre>';

$bdd->exec("... VALUES('".$ip_user."' ... ");

$bdd->exec('INSERT INTO ips(ip, c{$identifiant}) VALUES("'.$ip_user.'", 1)');

$bdd->exec("INSERT INTO ips(ip, c{$identifiant}) VALUES("'.$ip_user.'", 1)");

$reponse1 = $bdd->query('SELECT * FROM ips WHERE ip="'.$ip_user.'" AND c1=1');

$reponse1 = $bdd->query("SELECT * FROM ips WHERE ip="'.$ip_user.'" AND c1=1");