Pass Variable from Javascript to PHP after function is executed

Put this under your Storage rules if you dont require users authenticating:

service firebase.storage {
  match /b/{bucket}/o {
    match /{allPaths=**} {
      allow read, write: if request.auth == null;
    }
  }
}

This goes into your view file index.html or index.php :

    <form action="" method="post" onsubmit="myFunction();return false;">

        <div class="form-group">
            <label for="title">Title:</label>
            <p class="lead emoji-picker-container">
                <input  value=""  type="text" required="" class="form-control" id="title" placeholder="Enter Notification Title" name="title">
            </p>

        </div>
        <div class="form-group">
            <label for="message">Message:</label>
            <p class="lead emoji-picker-container">
                <textarea  required="" class="form-control" rows="5" id="message" placeholder="Enter Notification Message" name="message">  </textarea>

            </p>
        </div>

        <div class="form-group"id="image_url_group">
            <label for="image_url">Image URL:</label>
            <input type="file" class="form-control" id="image_url" placeholder="Enter Image URL" name="image_url" accept=".xlsx,.xls,image/*,.doc, .docx,.ppt, .pptx,.txt,.pdf" >
        </div>

        <button type="submit" class="btn btn-info">Submit</button>
    </form>

AJAX Submitting image URL to php, This goes into your view file index.html or index.php :

<script src="https://www.gstatic.com/firebasejs/7.14.4/firebase-app.js"></script>

<!-- TODO: Add SDKs for Firebase products that you want to use
     https://firebase.google.com/docs/web/setup#available-libraries -->
<script src="https://www.gstatic.com/firebasejs/7.14.4/firebase-analytics.js"></script>

<!-- Add Firebase products that you want to use -->
<script src="https://www.gstatic.com/firebasejs/7.14.4/firebase-auth.js"></script>
<script src="https://www.gstatic.com/firebasejs/7.14.4/firebase-firestore.js"></script>
<script src="https://www.gstatic.com/firebasejs/7.14.4/firebase-database.js"></script>
<script src="https://www.gstatic.com/firebasejs/7.14.4/firebase-storage.js"></script>

<script>

    // Your web app's Firebase configuration
    var firebaseConfig = {
        apiKey: "YOUR_API_KEY",
        authDomain: "YOUR_AUTH_DOMAIN",
        databaseURL: "YOUR_DATABASE_URL",
        projectId: "YOUR_PROJECT_ID",
        storageBucket: "YOUR_STORAGE_BUCKET",
        messagingSenderId: "YOUR_SENDER_ID",
        appId: "YOUR_APP_ID",
        measurementId: "YOUR_MEASUREMENT_ID"
    };


    // Initialize Firebase
    firebase.initializeApp(firebaseConfig);
    firebase.analytics();

    function AJAXSubmit (image_url) {
        //if (!oFormElement.action) { return; }
        var oReq = new XMLHttpRequest();

        if (image_url) {
            oReq.open("post", 'ajax.php');

            oReq.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");

            oReq.onreadystatechange = function() { // Call a function when the state changes.
                if (this.readyState === XMLHttpRequest.DONE && this.status === 200) {
                    console.log("Success finished");
                }
            }
            oReq.send("image_url=" + image_url);

            //oReq.onload = ajaxSuccess; //callback once AJAX is done
        }
}

    function myFunction(){
        var messagesRef = firebase.database().ref('Notification');
        //firebase  storage reference
        //it is the path where yyour image will store
        var imageName = 'WhateverYouWantToName';
        var storageRef=firebase.storage().ref('Notification/' + imageName);
        //upload image to selected storage reference

        const selectedFile = document.getElementById('image_url').files[0];

        var uploadTask=storageRef.put(selectedFile);

        uploadTask.on('state_changed',function (snapshot) {
            //observe state change events such as progress , pause ,resume
            //get task progress by including the number of bytes uploaded and total
            //number of bytes
            var progress=((snapshot.bytesTransferred/snapshot.totalBytes)*100).toFixed(0);

            console.log("upload is " + progress +" done");
        },function (error) {
            //handle error here
            console.log(error.message);
        },function () {
            //handle successful uploads on complete

            uploadTask.snapshot.ref.getDownloadURL().then(function (downlaodURL) {

// I want To Pass this variable to PHP in Image_url
                var ImageLink    = downlaodURL;
                AJAXSubmit(ImageLink);
                console.log(ImageLink);
            });
        });
    }

</script>

My firebaseConfig (this is my sandbox test firebase object) object is, make sure you get your correct credentials (example below) from your firebase project for this to work :

var firebaseConfig = {
    apiKey: "AIzaSyAvBKb-l7n5ixDSo2py-iN7m4nwFTKJ3rY",
    authDomain: "justtesting-aa1b4.firebaseapp.com",
    databaseURL: "https://justtesting-aa1b4.firebaseio.com",
    projectId: "justtesting-aa1b4",
    storageBucket: "justtesting-aa1b4.appspot.com",
    messagingSenderId: "258097526144",
    appId: "1:258097526144:web:a65d0cfd9b315a8071fd65",
    measurementId: "G-QWS4MW056K"
};

Create a file named ajax.php to receive the image_url :

if(isset($_POST['image_url'])){
    $fh = fopen( 'success.txt', 'a');
    fwrite($fh, $_POST['image_url'] );
    fclose($fh);
    exit;
}

See screenshot results of this working :