<?php
if(isset($_POST['submit']))
{
    if (empty($_POST["firstName"])) {
        $Err[] = "* First Name is required";
    } else {
        $name = test_input($_POST["firstName"]);
        // check if name only contains letters and whitespace
        if (!preg_match("/^[a-zA-Z ]*$/",$name)) {
            $Err[] = "Only letters are allowed in First Name";
        }
    }

    if (empty($_POST["email"])) {
        $Err[] = "* Email is required";
    } else {
        $email = test_input($_POST["email"]);
        // check if e-mail address syntax is valid
        if (!preg_match("/([w-]+@[w-]+.[w-]+)/",$email)) {
            $Err[] = "Invalid email format";
        } else {
            $emailSQL = "SELECT email FROM userdetails WHERE email = ?";
            $SQ = $conn->prepare($emailSQL) or die("ERROR: " . implode(":", $conn->errorInfo()));

            $SQ->bindParam(1, $email);
            $SQ->execute();
            $result = $SQ->fetch();
            if($result > 0) {
                $Err[] = "Sorry, email is already in use by another user";
            }
        }
    }

    if (empty($_POST["surname"])) {
        $Err[] = "* Surname is required";
    } else {
        $surname = test_input($_POST["surname"]);
        // check if name only contains letters and whitespace
        if (!preg_match("/^[a-zA-Z ]*$/",$name)) {
            $Err[] = "Only letters are allowed in Surname";
        }
    }
    if (empty($_POST["password"])) {
        $Err[] = "* Password is required";
    } else {
        $password = test_input($_POST["password"]);
    }
    if (empty($_POST["userName"])) {
        $Err[] = "* Username is required";
    } else {
        $username = test_input($_POST["userName"]);
        // check if name only contains letters and whitespace
        if (!preg_match("/^[a-zA-Z ]*$/",$name)) {
            $Err[] = "Only letters are allowed in Username";
        }
    }
}
?>

<form>Full name:&nbsp;
<input type="text" name="name" /><span class="error" id="nameError"></span>;
Email Address:
<input id="emailCheck" type="text" name="email" /><span class="error" id="emailError">
</form>

//add in your connection to the database here, mine is just a require_once statement
//get the passed parameter
$email = mysql_real_escape_string(strtolower($_POST["email"]));
//send a request to the database
$sql = "SELECT email FROM your_dbtable_name WHERE LOWER(email) = '" . $email . "'";
$result = mysql_query($sql, $conn) or die("Could not get email: " . mysql_error());
if(mysql_num_rows($result) > 0) {
//email is already taken
echo 0;
}
else {
//email is available
echo 1;
}

<script src="http://code.jquery.com/jquery-1.10.1.min.js"></script>
$(document).ready(function() {
//turns off autocomplete for browsers on email field
$('#emailCheck').attr('autocomplete', 'off');

//listens for typing on the desired field
$("#emailCheck").keyup(function() {
//gets the value of the field
var email = $("#emailCheck").val();
//displays a loader while it is checking the database
$("#emailError").html("<img alt="" src="../images/loader.gif" />");
//here is where you send the desired data to the PHP file using ajax
$.post("../php/checkavailability.php", {email:email},
function(result) {
if(result == 1) {
//the email is available
$("#emailError").html("Available");
}
else {
//the email is not available
$("#emailError").html("Email not available");
}
});
}
});
});