SOAP Webservice - error on line 2 at column 1: Extra content at the end of the document

Question

I'm trying to create a user-defined SOAP Webservice using nusoap with the following codes:

service.php

<?php
require 'nusoap/lib/nusoap.php';
require 'fetchdata.php';

$server = new nusoap_server(); 
$server->configureWSDL("Book Info Service","urn:soaptest"); 
$server->register(
    "get_info", 
    array("input"=>"xsd:string"),  
    array("return"=>"xsd:string")  
);


$server->service(file_get_contents("php://input"));

?>

fetchdata.php

<?php

function get_info($input)
{
    $books = [
        "9780435910105"=>["Opening Spaces: An Anthology of Contemporary African Women's Writing","Yvonne Vera","Heinemann"],
        "9780394721170"=>["African Folktales","Roger D. Abrahams","Knopf Doubleday Publishing Group"],
        "9781558615007"=>["Women Writing Africa: West Africa and the Sahel","Esi Sutherland-Addy","Feminist Press at CUNY"]
    ];
    
    foreach($books as $isbn=>$info)
    {
        if($isbn==$input)
        {
            return "<b>Title:</b> ".$info[0]."<br /><br /><b>Author:</b> ".$info[1]."<br /><br /><b>Publisher:</b> ".$info[2];
            break;
        }
                                    
    }
        return "No information found for"." ".$input;        
}

?>

When I run it locally, the WSDL page throws an error and the rest of the page is blank

This page contains the following errors:

error on line 2 at column 1: Extra content at the end of the document

Below is a rendering of the page up to the first error.

Why is this happening? I want to integrate this into an app so I need the WSDL link.

I honestly just copied and modified this so I don't really know what is happening. Can anyone help? Thank you.