用PHP自定义文本图像

I have some questions.. Whenever I try to edit text generated over this image, everything stops working. This is my raw version of the code. How can I change font style and size? $fontSize doesn't work and if I add something like $fontPath = 'URL'; and add $fontPath to imagestring, everything stops working too. How can I automatically center text with $x?

index.php

<!DOCTYPE html>
 <html>
  <head>
   <meta charset="utf-8">
   <title>Example</title>
  </head>
  <body>
   <h1>Example</h1><br/>
   <img src="image.php"/> 
  </body>
 </html>

image.php

<?php
header("Content-type: image/jpeg");
$imgPath = 'https://i.imgur.com/GvB85NN.jpg';
$image = imagecreatefromjpeg($imgPath);
$color = imagecolorallocate($image, 255, 255, 255);
$string = "test";
$fontSize = 20;
$x = 185;
$y = 185;
imagestring($image, $fontSize, $x, $y, $string, $color);
imagejpeg($image);
// $font_file
?>