PHP file does not get "POST' value from the Angular 10 page

Question

So I have bee trying to make a simple login page using Angular 10 as frontend and PHP as a server-side backend language(no PHP frameworks used here).

Here is my login.service.ts file:

---

    import { Injectable } from '@angular/core';
    import { HttpClient, HttpErrorResponse, HttpHeaders, HttpParams} from '@angular/common/http';

    //import { User } from '../shared/user.model';
    import { throwError } from 'rxjs';
    //import { map, tap } from 'rxjs/operators'

    export interface LoginResponse{
        id:bigint;
        username:string;    
        password:string
        user_date:string    
        show_user:number; 
        save_time:string
    }

    @Injectable({
        providedIn:'root'
    })


    export class LoginService{

        constructor(private http: HttpClient){}


        loginUser(username: string, password:string){
        
            const user = JSON.stringify( { "user" : {"username":username, "password" : password} });
            return this.http.post<any>(
                "http://localhost/calibration/login.php",
                user,
                {
                    headers: new HttpHeaders({'Content-Type':'text/plain'})
                }
            );
        }

        handleError(errorResponse:HttpErrorResponse){
            console.log(errorResponse);
            return throwError("Something Went Wrong");
        }
            
    }

and here is my login.php file:

        <?php  
        session_start();
        require_once("connection.php");
        var_dump($conn);
        try  
        {  
        $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);  
        var_dump($_POST);
        if(isset($_POST["user"]))  
        {     
                $obj = json_decode($_POST["user"]);
                $query = "SELECT * FROM user_master WHERE username = ? AND password = ?";  
                $statement = $conn->prepare($query); 
                $statement->bind_param(1, $username, $_POST['username']);
                $statement->bind_param(2, $password, $_POST['password']);
                $username = $obj->username;
                $password = $obj->password;
                $statement->execute();  
                $count = $statement->rowCount();  
                if($count > 0)  
                {  
                        $_SESSION["username"] = $_POST["username"];  
                        //header("location:login_success.php");  
                }  
                else  
                {  
                        $message = '<label>Wrong Data</label>';  
                }  
        }
        }  
        catch(PDOException $error)  
        {  
        $message = $error->getMessage();  
        }

    ?>   

Here is the response I received after trying to run the PHP file through the login.service.ts:

            {
        "headers": {
        "normalizedNames": {},
        "lazyUpdate": null
        },
        "status": 200,
        "statusText": "OK",
        "url": "http://localhost/calibration/login.php",
        "ok": false,
        "name": "HttpErrorResponse",
        "message": "Http failure during parsing for http://localhost/calibration/login.php",
        "error": {
        "error": {},
        "text": "object(PDO)#1 (0) {\n}\narray(0) {\n}\n  "
        }
    }

    text: "object(PDO)#1 (0) {\n}\narray(0) {\n}\n

Any help?

Update: Made some changes to the code to reflect some of the recommendations given below