Trying to add $var to a URL in a Curl URL - php

Question

So I have looked through stack overflow and google and cant find a good example of what I need.
I have a URL within Curl and i have a session variable i need to add to the URL (print out as string).
Question
how do i do this?
Here is my code
<?php
session_start();
echo $_SESSION['userID'];
$userId = $_SESSION['userID'];
$service_url = 'http://localhost/app/api/user/$userId.xml';
I have tried
$service_url = 'http://localhost/app/api/user/echo $userId.xml';
and
$service_url = 'http://localhost/app/api/user/<?php echo $userId ?>.xml';
UPDATE
Ok full code, so if I change the URL to
$service_url = 'http://localhost/app/api/user/1.xml';
everything works, however I need the code to be dynamic depending on the session var
currently the session var $_SESSION['userID']; is 1
so the current code should work in the same way as if the URL was
`$service_url = 'http://localhost/app/api/user/1.xml';`
but it dose not
<?php
session_start();
echo $_SESSION['userID'];
$userId = $_SESSION['userID'];
$service_url = "http://localhost/app/api/user/{$userId}.xml";
$session_cookie = file_get_contents( 'session_cookie.txt' );
// set up the request
$curl = curl_init( $service_url );
curl_setopt( $curl, CURLOPT_RETURNTRANSFER, true ); // have curl_exec return a string
curl_setopt( $curl, CURLOPT_COOKIE, "$session_cookie" ); // use the previously saved session
// make the request
curl_setopt( $curl, CURLOPT_VERBOSE, true ); // output to command line
$response = curl_exec( $curl );
curl_close( $curl );
print "RESPONSE:\n";
var_dump( $response );
$xml=simplexml_load_file("$service_url") or die("Error: Cannot create object");
echo $xml->name . "<br>";
echo $xml->roles->item . "<br>";
echo $xml->timezone . "<br>";
Instead I get errors
Warning: simplexml_load_file
and
Warning: simplexml_load_file(): I/O warning : failed to load external entity "http://localhost/app/api/user/1%3Cbr%3E.xml" in C:\wamp64\www\mytest\testfolder\authenticated.php on line 23

Answer 1

Try
$service_url = "http://localhost/app/api/user/{$userId}.xml";

Answer 2

Change ' to ".
Let me explain why.
The difference ' to " is that " will parse $phpVariables.
So $service_url = "http://localhost/app/api/user/$userId.xml"; or $service_url = "http://localhost/app/api/user/{$userId}.xml";
If for some reason you want to keep single quotes you can do this:
$service_url = 'http://localhost/app/api/user/'.$userId.'.xml';

Answer 3

Why not CONCAT them?
$service_url = 'http://localhost/app/api/user/'.$userId.'.xml';