$data = array("A", "B", 9, "12", "D");
$sql  = "INSERT INTO table (f) VALUES";

foreach($data as $val) {
    if (is_numeric($val)) {
        $sql .= "($val),";
    } else {
        $sql .= "('$val'),";
    }
}
$sql = substr($sql, 0, -1);

print($sql); // INSERT INTO table (f) VALUES('A'),('B'),(9),(12),('D')