if(isset($_POST)){

    $username = $_POST["username"];
    $email = $_POST["email"];
    $password = $_POST["password"];

    $sql = 'insert into users(username, email, password) values("' . $username . '", "' . $email . '", "' . $password . '")';

    if($conn->query($sql) === true){
        $output = "Inserted " . $username;

        echo json_encode($output);
    } else{
        echo json_encode("Error: " . $conn->error);
    }

}

writeTable(u, e, p) : Promise<any>{

let url = "http://localhost/gameon-server-side/create.php";

let param = { username: u, email: e, password: p};

let request = this.http.post(url, param);

return request.toPromise();

<ion-row>
    <ion-col text-center>

        <p>Response:</p>
        <p>{{responseTxt}}</p>

        <button ion-button (click)="showTable()"> Read from table </button>

        <ion-input class="txt" type="text" placeholder="Enter Username" [(ngModel)]="userName"></ion-input>
        <ion-input class="txt" type="text" placeholder="Enter Email" [(ngModel)]="userEmail"></ion-input>
        <ion-input class="txt" type="text" placeholder="Enter Password" [(ngModel)]="userPassword"></ion-input>

        <button ion-button (click)="addTable(userName, userEmail, userPassword)"> Insert </button>
        <button ion-button> Update </button>
        <button ion-button> Delete </button>

    </ion-col>
</ion-row>

addTable(u, e, p){
this.network.writeTable(u, e, p)
.then(data => {
  console.log("I received: " + JSON.stringify(data));
  this.responseTxt = ""+ JSON.stringify(data);
})
.catch(e => {
console.log(e);