Php 如何在此表单中添加reCaptcha？

我想在这个表单中添加Google reCaptcha我尝试了所有的方法，但都没有成功。。我需要做什么才能使表单成功提交。。还可以使用Ajax表单提交将reCaptcha设置为必填字段

HTML:-

<form id="contact-form" class="uk-form">
                        <fieldset>
                            <label>Name <em>*</em></label>
                            <input type="text" name="name" class="textfield" id="name" value="" />
                            <label>E-mail <em>*</em></label>
                            <input type="email" name="email" class="textfield" id="email" value="" />
                            <label>Services <em>*</em></label>
                            <input type="text" name="subject" class="textfield" id="subject" value="" />
                            <label>Message <em>*</em></label>
                            <textarea name="message" id="message" class="textarea" cols="2" rows="6"></textarea>
                            <div class="clearfix">
                                <div class="g-recaptcha" data-sitekey="My_Site_Key"></div>
                            </div>
                            <label>&nbsp;</label>
                            <button type="submit" name="submit" class="uk-button uk-button-small idz-button-blue" id="buttonsend">Submit</button>
                            <span class="loading" style="display: none;">Please wait..</span>
                        </fieldset>
                    </form>

$to = "xyz@mydomain.com"; 
 $subject = "Contact Inquiry";
 $name = $_POST['name'];
 $email = $_POST['email'];
 $services = $_POST['subject'];
 $message = $_POST['message'];
 $str = "Name: ".$name."\r\n\r\n"."Services: ".$services."\r\n\r\n"."Message: ".$message."\r\n\r\n".
 $headers = "From: $email"; 
 $sent = mail($to, $subject, $str, $headers) ; 

$to = "xyz@mydomain.com"; 
 $subject = "Contact Inquiry";
 $name = $_POST['name'];
 $email = $_POST['email'];
 $services = $_POST['subject'];
 $message = $_POST['message'];
 $str = "Name: ".$name."\r\n\r\n"."Services: ".$services."\r\n\r\n"."Message: ".$message."\r\n\r\n".
 $headers = "From: $email"; 
 $sent = mail($to, $subject, $str, $headers) ;