如何在Angular4中使用PHP会话来检查用户是否登录？

<?php
session_start();

if(isset($_SESSION['user_id'])){
    // 用户已登录
    echo json_encode(array('loggedIn' => true));
} else {
    // 用户未登录
    echo json_encode(array('loggedIn' => false));
}
?>

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';

@Injectable({
  providedIn: 'root'
})
export class AuthService {
  constructor(private http: HttpClient) { }

  checkLogin() {
    return this.http.get<any>('path/to/check-login.php');
  }
}

import { Component } from '@angular/core';
import { AuthService } from './auth.service';

@Component({
  selector: 'app-login-check',
  template: `
    <div *ngIf="loggedIn; else notLoggedIn">
      用户已登录
    </div>
    <ng-template #notLoggedIn>
      用户未登录
    </ng-template>
  `
})
export class LoginCheckComponent {
  loggedIn: boolean;

  constructor(private authService: AuthService) { }

  ngOnInit() {
    this.authService.checkLogin().subscribe(response => {
      this.loggedIn = response.loggedIn;
    });
  }
}