PHP Form with Captcha doesn't work (white screen) - php

Question

So the PHP contact form for my website doesn't work. No idea why. Tried many different templates. Only one worked for a while, before I tried to insert captcha there, and then it also started making 500 error.
No idea how PhP works from the word du tout - I just need a simple form for my personal website to have a captcha and that's all I ever want from PHP at this stage.
Below is the code that doesn't work (supposedly it should, as I have done it from this tutorial https://www.youtube.com/watch?v=4cr85kvM8I0
<?php
if(isset($_POST['submit'])){
$name = $_POST['name'];
$subject = $_POST['subject'];
$email_address = $_POST['email'];
$message = $_POST['message'];
$email_from = 'contact#mywebsite.com';
$email_subject = 'New email that you have received';
$email_body = "Name: $name.\n".
"Subject: $subject.\n".
"Email: $email_address.\n".
"Message: $message.\n";
$to_email = "contact#mywebsite.com";
$headers = "From: $email_from \r\n";
$headers .= "Reply-to: $email_address\r\n";
//your site secret key
$secretKey = 'mykey';
$responseKey = $_POST['g-captcha-response'];
$UserIP = $_SERVER['REMOTE_ADDR'];
$url = "https://www.google.com/recaptcha/api/siteverify?secret=$secretKey&response=$responseKey&remoteip=$UserIP";
$response = file_get_contents($url);
$response = json_decode($response);
if ($response-> success)
{
mailto($to_email,$email_subject,$email_body,$headers);
echo "Message Sent Successfully";
}
else {
echo "Invalid Captcha, try again";
}
}
?>
and this is the contact form (part of it on the html)
Head code:
<script src="https://www.google.com/recaptcha/api.js" async defer></script>
...
<div class="container">
<h3>Contact Form</h3>
<form action="contact-form-handler.php" method="POST"
enctype="multipart/form-data" name="contact_form">
<div class="nameemail">
<label for="name">Name</label>
<input name="name" type="text" required placeholder="Name"/>
<label for="subject">Subject</label>
<input name="subject" type="text" required placeholder="Subject"/>
<label for="email">Email</label>
<input name="email" type="email" required placeholder="you#domain.com"/>
</div>
<br>
<label for="message">Message</label><br>
<textarea name="message" cols="30" rows="10" placeholder="Enter your message here ..." required> </textarea>
<div class="center">
<div class="g-recaptcha" data-sitekey="mykey"></div>
<input type="submit" value="Submit">
</div>
</form>
</div>
So why in the mother of God this doesn't work?