save an IP in a database

I show you a code that I've tried in phpfiddle and it works.

I have applied some of the indications that your colleagues have made in comments, especially the queries prepared, to be able to enter your data safely.

Note: The second part of the code is just a test, to show that it works.

code

VIEW DEMO

<?php

require "util/public_db_info.php";

    $mysqli = new mysqli($host_name, $user_name, $pass_word, $database_name, $port);

    /*
        * Las consultas preparadas sustituyen los valores reales
        * por signos de interrogación en MySQLi
    */
    $sql = "INSERT INTO datos (correo, ip) VALUES (?, ?)";    

    /*
        * Almacenar en variables los datos a insertar
        * Cambia las variables puestas a mano 
        * por las variables obtenidas por $_POST
    */
    $correo= "[email protected]";  //$_POST['email'];
    $ip= get_client_ip();         //$_POST['ip'];;

    //Preparar la consulta
    $stmt=$mysqli->prepare($sql);

    //Evaluar si  la preparación tuvo  éxito
    if ($stmt) 
    {
         /*
           * Pasar parámetros separados  de la instrucción SQL
           * la letras "ss" indican el tipo de cada dato que se va a insertar
           * s: String, si es una cadena , i: Integer, si fuera un entero, etc
           * Ejecutar
        */
        $stmt->bind_param("ss", $correo,$ip);
        $stmt->execute();

        /*
          * Imprimir la cantidad de filas insertadas usando affected_rows
        */

        printf("%d Filas insertadas.\n", $stmt->affected_rows);

        /*
         * Cerrar $stmt y luego la conexión
         * para liberar recursos
        */

    $stmt->close();
}

    // Comprobar si se insertaron los datos
    $sql = "SELECT * FROM datos";   

    if ($resultado = $mysqli->query($sql)) 
    {
           echo "<pre>";

        while ($row = mysqli_fetch_array($resultado))  
        {
           echo $row["correo"]." : ".$row["ip"];
        }

            echo "</pre>";

        $resultado->close();

    } 
       else 
    {
        echo "No hay datos";
    }

$mysqli->close();



    //Obtiene la IP del cliente

    function get_client_ip() {
        $ipaddress = '';
        if (getenv('HTTP_CLIENT_IP'))
            $ipaddress = getenv('HTTP_CLIENT_IP');
        else if(getenv('HTTP_X_FORWARDED_FOR'))
            $ipaddress = getenv('HTTP_X_FORWARDED_FOR');
        else if(getenv('HTTP_X_FORWARDED'))
            $ipaddress = getenv('HTTP_X_FORWARDED');
        else if(getenv('HTTP_FORWARDED_FOR'))
            $ipaddress = getenv('HTTP_FORWARDED_FOR');
        else if(getenv('HTTP_FORWARDED'))
           $ipaddress = getenv('HTTP_FORWARDED');
        else if(getenv('REMOTE_ADDR'))
            $ipaddress = getenv('REMOTE_ADDR');
        else
            $ipaddress = 'UNKNOWN';
        return $ipaddress;
    }
    ?>

result

-1 Filas insertadas.

[email protected] : 148.3.73.185

You can do it in two different ways.

1- Assigning the variable via JAVASCRIPT, how you were already trying, but declaring the variables correctly.

<script type="text/javascript">//aqui no entra o no hace nada
   var ip = "<?php echo $ip; ?>";
   alert ("hola");
   alert(ip);
   document.getElementById("dir").value=ip;
</script>

2- Assigning the value directly in the field for this purpose.

<form method="post" action="altab.php">
    Correo:<br/>
    <input type="text" name="correo" />
    <input type="hidden" id="dir" name="ip" value="<?php echo $ip; ?>"> <!-- Podría ser <?=$ip?>, como fomra abreviada -->
    <input type="hidden"  name="fecha" > <!-- Aquí también puedes asignar el valor directamente al campo -->

    <input type="submit" value="listo">
</form>

Both forms are valid, but find the second most appropriate (personal opinion) because the assignment is direct and, unless you need to process this information with javascript, it is much easier.