PHP Function can't read jquery POST data?

Question

Alright so I have this function

<?php

/**
 * @author Mitchell A. Murphy
 * @copyright 2011
 */
include ('func_lib.php');
connect();
echo (check($_POST['input']) ? 'true' : 'false');
function check($args)
{
    $args = strtolower($args);
    $checkemail = "/^[a-z0-9]+([_\\.-][a-z0-9]+)*@([a-z0-9]+([\.-][a-z0-9]+)*)+\\.[a-z]{2,}$/i";
    if (preg_match($checkemail, $args))
    {
        //logic for email argument
        $sql = "SELECT * FROM `users` WHERE `email`='" . $args . "'";
        $res = mysql_query($sql) or die(mysql_error());
        echo "type=email:";
        if (mysql_num_rows($res) > 0)
        {
            return true;
        } else
        {
            return false;
        }
    } else
    {
        //logic for username argument
        $sql = "SELECT * FROM `users` WHERE `username`='" . $args . "'";
        $res = mysql_query($sql) or die(mysql_error());
        echo "type=username:";
        if (mysql_num_rows($res) > 0)
        {
            return true;
        } else
        {
            return false;
        }
    }

}

?>

The function should be accessed by this jquery script:

$('form.register .submit').click(validateRegister);

function validateRegister() {

    //Variables
    var emailExists = false;
    var userExists = false;
    var $error = "";

    //Executes functions
    email();


    function email() {
        var $error = $('#email .error');
        var input = $('#email input').val();
        var emailRE = /^.*@.+\..{2,5}$/;
        if (input.match(emailRE)) {
            $error
                .html('<div>Proper Email Format: <span>Hello@Yoursite.com</span></div>')
                .animate({
                'left': '-130px',
                'opacity': '0'
            });

            //Checks for Existing Email

            function checkExisting_email() {
                $.ajax({
                    type: 'POST',
                    url: 'includes/checkExist.php',
                    data: input,
                    statusCode: {
                        404: function () {
                            alert('page not found');
                        }
                    },

                    success: function (data) {
                        alert(data);
                    },
                    error: function () {
                        alert("error bro");
                    }
                });

            }
            emailExists = checkExisting_email();

            //If it exists
            if (emailExists) {
                alert("This email already exists!");
            } else if (emailExists == false) {
                alert("Email doesnt exist!");
            }

        } else {
            //Email doesn't match
            $error
                .html('<div>Proper Email Format: <span>Hello@Yoursite.com</span></div>')
                .animate({
                'left': '-150px',
                'opacity': '1'
            });
        }
    }
    return false;
}

But for some reason the script (js) isn't sending any data? if so, how do i reference it. I am the backend developer but the designer who did the javascript left me to fix this. I know the php works because I made a test form to send the data with this html markup:

<form action="includes/checkExist.php" method="post">
<input type="text" name="input" />
<input type="submit" name="submit" />
</form>

And that works...so why is the input from jquery returning as NULL?