Alert box not working in php

This is my contact-form-handler.php file. After clicking submit button, I want an alert in PHP. I am receiving Mail successfully and redirect to my main page but without an alert message. How can I use the alert box in PHP?

My form is working fine, but this time I stucked in this simple task.

<?php 
    if(isset($_POST['submit'])){
        //$companyMail = 'enesh@gmail.com';
        $to = "eneshpal@gmail.com"; // this is your Email address
        $customerMail = $_POST['formEmail']; // this is the sender's Email address
        $first_name = $_POST['formFirstName'];
        $last_name = $_POST['formLastName'];
        $phone = $_POST['formPhone'];
        $text = $_POST['formText'];
        /* foreach($_POST['project_type'] as $project_type_value){} */
        $projectType = implode(', ',$_POST['project_type']);
        $scopeProject = implode(', ',$_POST['scope_project']);
        $project_type_Str = 'Project Type : '.$projectType;
        $scope_project_Str = 'Scope of Project : '.$scopeProject;
        $subject = "Form Submission";
        $message = "Hi, \n\n";
        $message .= "First Name: ".$first_name . "\nLast Name: " . $last_name . " \nEmail: " . $customerMail . " \nPhone : " . $phone . "\nDescription: " . $text . "\n";
        $message .=  $project_type_Str."\n";
        $message .= $scope_project_Str;
        //$message2 = "Here is a copy of your message " . $first_name . "\n\n" . $_POST['message'];
        //$headers = "From:" . $companyMail;
        //$headers2 = "From:" . $to;
        if(mail($to,$subject,$message)){
            //echo 'Mail Sent';
            //$message = "Thanks, We Will Contact you Shortly";
            //echo "<script type='text/javascript'>alert('$message');</script>";
            echo "<script>alert('Thanks, We Will Contact you Shortly');</script>";
            header('Location:get_estimation.php');
        }else{
            echo 'Mail Not Sent';
        }
        //mail($from,$subject2,$message2,$headers2); // sends a copy of the message to the sender
        /*if( "Mail Sent. Thank you " . $first_name . ", we will contact you shortly.")
        else die; */
        // You can also use header('Location: thank_you.php'); to redirect to another page.
    }
?>