如何利用PHP函数进行数据存储和读取的数据校验？

$email = "test@example.com";
if(filter_var($email, FILTER_VALIDATE_EMAIL)){
    echo "邮箱地址格式正确";
}else{
    echo "邮箱地址格式错误";
}

$url = "http://www.example.com";
if(filter_var($url, FILTER_VALIDATE_URL)){
    echo "URL地址格式正确";
}else{
    echo "URL地址格式错误";
}

$ip = "192.168.1.1";
if(filter_var($ip, FILTER_VALIDATE_IP)){
    echo "IP地址格式正确";
}else{
    echo "IP地址格式错误";
}

$number = "123";
if(is_numeric($number)){
    echo "输入的值是一个整数";
}else{
    echo "输入的值不是一个整数";
}

$password = "123456";
if(strlen($password) < 6){
    echo "密码长度太短";
}else{
    echo "密码长度合适";
}

$username = "test";
// 假设连接数据库的代码已经完成
$sql = "SELECT * FROM users WHERE username = '$username'";
$result = mysqli_query($connection, $sql);
if(mysqli_num_rows($result) > 0){
    echo "用户名已存在";
}else{
    echo "用户名可用";
}