<?php $email;$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($responseKeys["success"]) !== 1) {
echo '<h2>You are spammer ! Get the @$%K out</h2>';
} else {
echo '<h2>Thanks for posting comment.</h2>';
}

<form class="contact_form" action="mail.php" method="post" name="contact_form">
<ul>
<li>
<input name="name" type="text" placeholder="Name" required />
<input name="e_mail" type="text" placeholder="Phone" required />
</li>
<li>
<textarea name="text" cols="40" rows="6" placeholder= "Text..." required ></textarea>
</li>
<li>
<div class="g-recaptcha" data-sitekey="**********"></div>
</li>
<li>
<button class="submit" type="submit">Send</button>
</li>
</ul>
</form>