php/mysql query

<?php
include_once("connect.php");
if (isset($_POST['action'])&&$_POST['action'] == "check") {
    $email = mysql_real_escape_string(strip_tags($_POST['email']));
    $username = mysql_real_escape_string(strip_tags($_POST['username']));

    if (strpos($email,"@")!==false) {
        $check_email = explode("@",$email);
        if (strpos($check_email[1],".")===false) {
            echo 3;
            exit();
        }
    } else {
        echo 3;
        exit();
    }
    $email_query = mysql_query("SELECT id FROM users WHERE email='$email' LIMIT 1");
    if (mysql_num_rows($email_query)== 1) {
        //Mail alredy exist
        echo 1;
        exit();
    } else {
        $username_query = mysql_query("SELECT username FROM users WHERE username='username' LIMIT 1");
        if (mysql_num_rows($username_query)== 1) {
            //name alredy exist
            echo 2;
            exit();
        } else {
            //evritin ok
            echo 0;
            exit();
        }
    }
}
if (isset($_POST['action'])&&$_POST['action']=="register") {
    $username = mysql_real_escape_string(strip_tags($_POST['username']));
    $password = mysql_real_escape_string(strip_tags($_POST['password']));
    $email = mysql_real_escape_string(strip_tags($_POST['email']));

    $activation_code = md5($username.$password);

    $reg_query = mysql_query("INSERT INTO users (username, password, email, activation_code) VALUES ('$username','".md5($password)."','$email','$activation_code')");
    if($reg_query) {
        //user folders, act mail
        $new_userid = mysql_insert_id();

        //Usur fuldor
        mkdir("../users/$new_userid",0755);
        mkdir("../users/$new_userid/images",0755);
        mkdir("../users/$new_userid/media",0755);

        $to = $email;
        $from = "kakaradarkorbit69@gmail.com";
        $subject = "Account activation";

        $message = "<h3>Welcum!</h3><p>Activate ur acc by clikin a link</p><p><a href='localhost?id=$new_userid&activate=$activation_code'>localhost?id=$new_userid&activate=$activation_code</a></p>";

        $headers = 'MIME-Version: 1.0'."\r\n";
        $headers .= 'Content-Type: text/html; charset=iso-8859-1'."\r\n";
        $headers .= "From: $from\r\nReply-To: $from";
        mail($to,$subject,$message,$headers);

        echo 1;
        exit();
    } else {
        //reg failed
        echo 0;
        exit();
    }
}
?>