Email form is refreshing page

I am trying to make a quotation form which I have managed to complete but I am really struggling with the php side of things. I have borrowed some code online and tried to make it adapt with my form but it keeps refreshing the page, please if there is anyone out there that can help please?

Here is my code

<?php
/*
This first bit sets the email address that you want the form to be submitted to.
You will need to change this value to a valid email address that you can access.
*/
$webmaster_email = "jhugill94@gmail.com";

/*
This bit sets the URLs of the supporting pages.
If you change the names of any of the pages, you will need to change the values here.
*/
$feedback_page = "/Quotation.html";
$error_page = "/error_message.html";
$thankyou_page = "/thank_you.html";

/*
This next bit loads the form field data into variables.
If you add a form field, you will need to add it here.
*/
$senderFirst = $_REQUEST['senderFirst'] ;
$senderLast = $_REQUEST['senderLast'] ;
$senderNumber = $_REQUEST['senderNumber'] ;
$senderEmail = $_REQUEST['senderEmail'] ;
$issue_discription = $_REQUEST['issue_discription'] ;
$car_make = $_REQUEST['car_make'] ;
$car_model = $_REQUEST['car_model'] ;
$fuel_type = $_REQUEST['fuel_type'] ;
$engine_size = $_REQUEST['engine_size'] ;
$image_upload = $_REQUEST['image_upload'] ;
$msg = 
"First Name: " . $senderFirst . "\r\n" . 
"Sur Name: " . $senderLast . "\r\n" . 
"Contact Number: " . $senderNumber . "\r\n" . 
"Email: " . $senderEmail . "\r\n" . 
"Issue Discription: " . $issue_discription ;
"car_make: " . $car_make ;
"car_model: " . $car_model ;
"fuel_type: " . $fuel_type ;
"engine_size: " . $engine_size ;
"image_upload: " . $image_upload ;

/*
The following function checks for email injection.
Specifically, it checks for carriage returns - typically used by spammers to inject a CC list.
*/
function isInjected($str) {
    $injections = array('(\n+)',
    '(\r+)',
    '(\t+)',
    '(%0A+)',
    '(%0D+)',
    '(%08+)',
    '(%09+)'
    );
    $inject = join('|', $injections);
    $inject = "/$inject/i";
    if(preg_match($inject,$str)) {
        return true;
    }
    else {
        return false;
    }
}

// If the user tries to access this script directly, redirect them to the feedback form,
if (!isset($_REQUEST['email_address'])) {
header( "Location: $feedback_page" );
}

// If the form fields are empty, redirect to the error page.
elseif (empty($first_name) || empty($email_address)) {
header( "Location: $error_page" );
}

/* 
If email injection is detected, redirect to the error page.
If you add a form field, you should add it here.
*/
elseif ( isInjected($email_address) || isInjected($first_name)  || isInjected($comments) ) {
header( "Location: $error_page" );
}

// If we passed all previous tests, send the email then redirect to the thank you page.
else {

    mail( "$webmaster_email", "Feedback Form Results", $msg );

    header( "Location: $thankyou_page" );
}
?>