ReCaptcha V2 returns False even though checked - php

Question

Having a problem with ReCaptcha. Even if checkbox is checked, it still returns false. I have triple checked my keys and happy they are correct. I am using code based on that successfully used on another of my sites. Cannot see what is wrong.
For ease of reference, I have copied all the relevant coding. Hopefully someone will see straight away what I've done wrong.
Form Code:
<form name="contactform" method="post" action="send_form_email.php">
<table width="450px">
<tr>
<td valign="top">
<label for="first_name">First Name *</label>
</td>
<td valign="top">
<input type="text" name="first_name" placeholder="Please enter your first name" maxlength="50" size="30">
</td>
</tr>
<tr>
<td valign="top"">
<label for="last_name">Last Name *</label>
</td>
<td valign="top">
<input type="text" name="last_name" placeholder="Please enter your surname" maxlength="50" size="30">
</td>
</tr>
<tr>
<td valign="top">
<label for="email">Email Address *</label>
</td>
<td valign="top">
<input type="email" name="email" placeholder="Please enter your email address" maxlength="80" size="30">
</td>
</tr>
<tr>
<td valign="top">
<label for="telephone">Telephone Number</label>
</td>
<td valign="top">
<input type="tel" name="telephone" placeholder="Please enter your phone number" maxlength="30" size="30">
</td>
</tr>
<tr>
<td valign="top">
<label for="comments">Comments *</label>
</td>
<td valign="top">
<textarea name="comments" maxlength="1000" cols="25" rows="6"></textarea>
</td>
</tr>
<tr>
<td colspan="2" style="text-align:center"><div class="g-recaptcha" data-sitekey="6LejdlwUAAAAAHb5qaNIhfDzxL1i9a9sCyw8dLzD"></div></td>
</tr>
<tr>
<td colspan="2" style="text-align:center">
<input type="submit" value="Submit"> <br> Email Form
</td>
</tr>
</table>
</form>
Code from the PHP:
<?php
// Code from ReCaptcha evaluation:
$email;$comment;$captcha;
if(isset($_POST['email'])){
$email=$_POST['email'];
}if(isset($_POST['comment'])){
$email=$_POST['comment'];
}if(isset($_POST['g-recaptcha-response'])){
$captcha=$_POST['g-recaptcha-response'];
}
if(!$captcha){
echo '<h2>Please check the the captcha form.</h2>';
exit;
}
$secretKey = "************************************";
$ip = $_SERVER['REMOTE_ADDR'];
$response=file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=".$secretKey."&response=".$captcha."&remoteip=".$ip);
$responseKeys = json_decode($response,true);
if(intval($responseKeys["success"]) !== 1) {
echo '<h2>You are a spammer ! Your form has been rejected.</h2>';
} else
// End Code from ReCaptcha evaluation.
Many Thanks
Tim