发送电子邮件的Web表单不起作用

我只需要一个非常基本/简单的表单来发送电子邮件，但它似乎不起作用，我不知道为什么？我知道代码不是很先进，可能看起来很"丑陋"，但它可以满足我对的"需求"

if (isset($_POST['email']))
{
    $full_name = $_POST['fullName'];
    $phone = $_POST['phoneNumber'];
    $adMessage = $_POST['emailMessage'];
    //send email
    $to = "info@lalalalala.co.uk";
    $from = $_POST['emailAddress'];
    $subject = $_POST['emailSubject'];
    $message = "Full name: " . $full_name . "'n";
    $message .= "Phone number: " . $phone . "'n";
    $message .= "Email Address: " . $from . "'n";
    $message .= "Additional Message: " . $adMessage;

    $headers = "From:" . $full_name;
    mail($to, $subject, $message, $headers);
}

HTML

<form action='' method='post'>
    <fieldset><label id="fullName">Full Name</label>
    <input type="text" name="fullName" value="Please Enter Your Full Name" onclick="this.value=''"  /></fieldset>
    <fieldset><label id="emailAddress">Email Address</label>
    <input type="text" name="emailAddress" value="Please Enter Your Email Address" onclick="this.value=''"  /></fieldset>
    <fieldset><label id="phoneNumber">Phone Number</label>
    <input type="text" name="phoneNumber" value="Please Enter Your Phone Number" onclick="this.value=''"  /></fieldset>
    <fieldset><label id="emailSubject">Email Subject</label>
    <input type="text" name="emailSubject" value="Please Enter Email Subject" onclick="this.value=''"  /></fieldset>
    <fieldset><label id="emailMessage">Your Message</label>
    <textarea cols="10" rows="20" name="emailMessage" onclick="this.value=''" >Please Enter Your Message</textarea></fieldset>
    <fieldset><input type="image" src="images/submit.png" id="submit" onclick="show_alert();" /></fieldset>
</form>

谢谢：）