Trying to get mail using PHP?

<?php
if($_POST)
{
    $to_email       = "hello@altitudequest.com"; //Recipient email, Replace with own email here
    $from_email       = $_POST["user_name"]; //From email, most emails won't work without it.

    //check if its an ajax request, exit if not
    if(!isset($_SERVER['HTTP_X_REQUESTED_WITH']) AND strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) != 'xmlhttprequest') {

        $output = json_encode(array( //create JSON data
            'type'=>'error', 
            'text' => 'Sorry Request must be Ajax POST'
        ));
        die($output); //exit script outputting json data
    } 

    //Sanitize input data using PHP filter_var().
    $user_name      = filter_var($_POST["user_name"], FILTER_SANITIZE_STRING);
    $user_email     = filter_var($_POST["user_email"], FILTER_SANITIZE_EMAIL);
    $phone_number   = filter_var($_POST["phone_number"], FILTER_SANITIZE_NUMBER_INT);
    $subject        = filter_var($_POST["subject"], FILTER_SANITIZE_STRING);
    $message        = filter_var($_POST["msg"], FILTER_SANITIZE_STRING);

    //additional php validation
    if(strlen($user_name)<4){ // If length is less than 4 it will output JSON error.
        $output = json_encode(array('type'=>'error', 'text' => 'Name is too short or empty!'));
        die($output);
    }
    if(!filter_var($user_email, FILTER_VALIDATE_EMAIL)){ //email validation
        $output = json_encode(array('type'=>'error', 'text' => 'Please enter a valid email!'));
        die($output);
    }

    if(!filter_var($phone_number, FILTER_SANITIZE_NUMBER_FLOAT)){ //check for valid numbers in phone number field
        $output = json_encode(array('type'=>'error', 'text' => 'Enter only digits in phone number'));
        die($output);
    }
    if(strlen($subject)<3){ //check emtpy subject
        $output = json_encode(array('type'=>'error', 'text' => 'Subject is required'));
        die($output);
    }
    if(strlen($message)<3){ //check emtpy message
        $output = json_encode(array('type'=>'error', 'text' => 'Too short message! Please enter something.'));
        die($output);
    }

    //email body
    $message_body = $message."\r\n\r\n-"
                    .$user_name."\r\nEmail : "
                    .$user_email."\r\nPhone Number : ". $phone_number ;

    //proceed with PHP email.
    $headers = 'From: '.$from_email.'' . "\r\n" .
    'Reply-To: '.$user_email.'' . "\r\n" .
    'X-Mailer: PHP/' . phpversion();

    $send_mail = mail($to_email, $subject, $message_body, $headers);

    if(!$send_mail)
    {
        //If mail couldn't be sent output error. Check your PHP email configuration (if it ever happens)
        $output = json_encode(array('type'=>'error', 'text' => 'Could not send mail! Please check your PHP mail configuration.'));
        die($output);
    }else{
        $output = json_encode(array('type'=>'message', 'text' => 'Hi '.$user_name .', your message has been sent. Thank you!'));
        die($output);
    }
}
?>

I am trying to send myself mail from an enquiry form, I am facing an issue that is

When I input my name in the form with a space for example: Rajat Audichya and submit the form I get email naming (unknown sender) and when I just put the first name that is Rajat without the space I get email as Rajat ?

What can I do to overcome this issue?