export class AppComponent {
public isValid:boolean;
constructor(private router:Router){
}
 logout(){
 localStorage.removeItem('email');
 this.router.navigate(['./login']);
 this.isValid=true;
 }
 }

export class LoginComponent implements OnInit {
public username:string;
public password:string;
constructor(private router:Router) { }
ngOnInit() {
}
userLogin(form:NgForm){
if(form.value.username==="admin@gmail.com" && form.value.password==="admin")
 {
  localStorage.setItem('email',form.value.username);
  this.router.navigate(['./php']);
  this.isValid=false;//not working//
} }  }

<ul class="nav navbar-nav navbar-right">
  <li *ngIf="isValid">
    <a [routerLink]="['/login']" >Login</a>
  </li>

  <li *ngIf="!isValid">
      <a (click)="logout()">LogOut</a>
    </li>

</ul>