Select row from database using PHP with Android interface

I want to select a row from my users’ table 'tbl_login' in my database with the following php script:

<?php

    include 'config.inc.php';

    $conn = mysqli_connect($servername, $username, $password, $dbname);

    $email = $_POST["email"];

    $sql = "SELECT email, firstname, lastname, location 
        FROM tbl_login
        WHERE email = '$email'";

    $res = mysqli_query($conn,$sql);

    $result = array();

    while($row = mysqli_fetch_array($res)){
        array_push($result,
        array('email'=>$row[0],
              'firstname'=>$row[1],
              'lastname'=>$row[2],
              'location'=>$row[3]
              ));
    }

    if ($conn->query($sql) === TRUE) {
        echo "Selected row successfully";
    } else {
        echo "Error: " . $sql . "<br>" . $conn->error;
    }

    echo json_encode(array("result"=>$result));

    mysqli_close($conn);

?>

My problem is that I’m not sure how to write this part:

$email = $_POST['email']; // <--- This.

$sql = "SELECT email, firstname, lastname, location 
        FROM tbl_login
        WHERE email = '$email'"; // <--- And this.

If I’m trying without the $_POST and with just a hardcoded string like 'Luke', it works, so I figured my problem is on the $_POST.

My others scripts are done this way and they all work, if you could enlighten me on why this is not working, I would be extremely grateful, as I've been working on this for a few hours already. Thanks a lot in advance, Charles.