How can I send push notification from PHP website to iOS and Android?

You can use several premade services for push notifications like Firebase Messaging, One Signal, etc. For Firebase cloud Messaging Integration with PHP, please visit this Link. After setting up, Just Send this POST request with the user token

POST https://fcm.googleapis.com/v1/projects/myproject-b5ae1/messages:send HTTP/1.1

Content-Type: application/json
Authorization: Bearer ya29.ElqKBGN2Ri_Uz...PbJ_uNasm

{
  "message": {
    "token" : <token of destination app>,
    "notification": {
      "title": "FCM Message",
      "body": "This is a message from FCM"
    },
    "webpush": {
      "headers": {
        "Urgency": "high"
      },
      "notification": {
        "body": "This is a message from FCM to web",
        "requireInteraction": "true",
        "badge": "/badge-icon.png"
      }
    }
  }
}

or Send a Push Notification using topic

https://fcm.googleapis.com//v1/projects/<YOUR-PROJECT-ID>/messages:send
Content-Type: application/json
Authorization: bearer <YOUR-ACCESS-TOKEN>

{
  "message": {
    "topic": "matchday"
    "notification": {
      "title": "Background Message Title",
      "body": "Background message body"
    },
    "webpush": {
      "fcm_options": {
        "link": "https://dummypage.com"
      }
    }
  }
}

For One Signal, Visit this Link for more information. Also, alternatively, you can use web sockets, if you don't want premade services.