Insert into MySQL database with PHP API

Been following this tutorial to learn how to create an API for android. Checked my DB_Functions.php file and everything is connecting and running correctly (90% sure). To make sure the post is working correctly I am using a chrome add-on called Postman. This other question I found online was having a similar problem to mine. This is what I inputted/received.
Here is the code

<?php

/* 
Function tests

require_once 'include/DB_Functions.php';
$db = new DB_Functions();

$insert = $db->storeUser("InsertTest", "[email protected]", "apple");
print_r($insert);

$user = $db->getUserByEmailAndPassword("[email protected]", "apple");
print_r($user);

$exist = $db->isUserExisted("[email protected]");
echo $exist; */

/**
* File to handle all API requests
* Accepts GET and POST
* 
* Each request will be identified by TAG
* Response will be JSON data

/**
* check for POST request 
*/

if (isset($_POST['tag']) && $_POST['tag'] != '') {
$tag = $_POST['tag'];

require_once 'include/DB_Functions.php';
$db = new DB_Functions();
$response = array("tag" => $tag, "error" => FALSE);

// check for tag type
if ($tag == 'login') {
    // Request type is check Login
    $email = $_POST['email'];
    $password = $_POST['password'];

    // check for user
    $user = $db->getUserByEmailAndPassword($email, $password);
    if ($user != false) {
        // user found
        $response["error"] = FALSE;
        $response["uid"] = $user["unique_id"];
        $response["user"]["name"] = $user["name"];
        $response["user"]["email"] = $user["email"];
        $response["user"]["created_at"] = $user["created_at"];
        $response["user"]["updated_at"] = $user["updated_at"];
        echo json_encode($response);
    } else {
        // user not found
        // echo json with error = 1
        $response["error"] = TRUE;
        $response["error_msg"] = "Incorrect email or password!";
        echo json_encode($response);
    }
} else if ($tag == 'register') {
    // Request type is Register new user
    $name = $_POST['name'];
    $email = $_POST['email'];
    $password = $_POST['password'];

    // check if user is already existed
    if ($db->isUserExisted($email)) {
        // user is already existed - error response
        $response["error"] = TRUE;
        $response["error_msg"] = "User already existed";
        echo json_encode($response);
    } else {
        // store user
        $user = $db->storeUser($name, $email, $password);
        if ($user) {
            // user stored successfully
            $response["error"] = FALSE;
            $response["uid"] = $user["unique_id"];
            $response["user"]["name"] = $user["name"];
            $response["user"]["email"] = $user["email"];
            $response["user"]["created_at"] = $user["created_at"];
            $response["user"]["updated_at"] = $user["updated_at"];
            echo json_encode($response);
        } else {
            // user failed to store
            $response["error"] = TRUE;
            $response["error_msg"] = "Error occured in Registartion";
            echo json_encode($response);
        }
    }
} else {
    // user failed to store
    $response["error"] = TRUE;
    $response["error_msg"] = "Unknown 'tag' value. It should be either      'login' or 'register'";
    echo json_encode($response);
}
} else {
$response["error"] = TRUE;
$response["error_msg"] = "Required parameter 'tag' is missing!";
echo json_encode($response);
} 
?>

And the DB_Functions.php code

<?php

class DB_Functions{

private $db;
public $connection;

function __construct(){
    require_once ('DB_Connect.php');
    $this->db = new DB_Connect();
    $this->connection = $this->db->connect();
}

function __destruct(){

}

public function storeUser($name, $email, $password){
    $uuid = uniqid('', true);
    $hash = $this->hashSSHA($password);
    $encrypted_password = $hash["encrypted"];
    $salt = $hash["salt"];
    $sql = "INSERT INTO users(unique_id, name, email, encrypted_password, salt, created_at) 
    VALUES ('$uuid', '$name', '$email', '$encrypted_password', '$salt', NOW())";
    $result = $this->connection->query($sql);

    if($result){
        $uid = mysqli_insert_id($this->connection);
        $sql = "SELECT * FROM users WHERE uid = '" . $uid . "';";
        $result = $this->connection->query($sql);
        return mysqli_fetch_array($result); 
    }else{
        return false;
    }
}

public function getUserByEmailAndPassword($email, $password){
    $sql = "SELECT * FROM users WHERE email = '" . $email . "';";
    $result = $this->connection->query($sql);
    $no_of_rows = mysqli_num_rows($result);

    if($no_of_rows > 0){
        $result = mysqli_fetch_array($result);
        $salt = $result['salt'];
        $encrypted_password = $result['encrypted_password'];
        $hash = $this->checkhashSSHA($salt, $password);

        if($encrypted_password == $hash){
            return $result;
        }
    }else{
        return false;
    }
}

public function isUserExisted($email){
    $sql = "SELECT * FROM users WHERE email = '" . $email . "';";
    $result = $this->connection->query($sql);
    $no_of_rows = mysqli_num_rows($result);

    if($no_of_rows > 0){
        return true;
    }else{
        return false;
    }
}

public function hashSSHA($password){
    $salt = sha1(rand());
    $salt = substr($salt, 0, 10);
    $encrypted = base64_encode(sha1($password . $salt, true) . $salt);
    $hash = array("salt" => $salt, "encrypted" => $encrypted);
    return $hash;
}

public function checkhashSSHA($salt, $password){
    $hash = base64_encode(sha1($password . $salt, true) . $salt);
    return $hash;
}

}
?>

Does anyone know why the Post isn't working?