Is preventing XSS and SQL Injection as easy as does this

Question: Is preventing XSS (cross-site scripting) as simple using strip_tags on any saved input fields and running htmlspecialchars on any displayed output ... and preventing SQL Injection by using PHP PDO prepared statements?

Here's an example:

// INPUT: Input a persons favorite color and save to database
// this should prevent SQL injection ( by using prepared statement)
// and help prevent XSS  (by using strip_tags)
$sql = 'INSERT INTO TABLE favorite (person_name, color) VALUES (?,?)';
$sth = $conn->prepare($sql);
$sth->execute(array(strip_tags($_POST['person_name']), strip_tags($_POST['color'])));


// OUTPUT: Output a persons favorite color from the database
// this should prevent XSS (by using htmlspecialchars) when displaying
$sql = 'SELECT color FROM favorite WHERE person_name = ?';
$sth = $conn->prepare($sql);
$sth->execute(array(strip_tags($_POST['person_name'])));
$sth->setFetchMode(PDO::FETCH_BOTH);
while($color = $sth->fetch()){
  echo htmlspecialchars($color, ENT_QUOTES, 'UTF-8');
}

It's even more simple. Just htmlspecialchars() (with quote style and character set) on user-controlled input is enough. The strip_tags() is only useful if you already want to sanitize data prior to processing/save in database, which is often not used in real world. HTML code doesn't harm in PHP source, but PHP code may do so if you use eval() on non-sanitized user-controlled input or that kind of evil stuff.

This however doesn't save you from SQL injections, but that's another story.

Update: to get clean user input from the request to avoid magic quotes in user-controlled input, you can use the following function:

function get_string($array, $index, $default = null) {
    if (isset($array[$index]) && strlen($value = trim($array[$index])) > 0) {
         return get_magic_quotes_gpc() ?  stripslashes($value) : $value;
    } else {
         return $default;
    }
}

which can be used as:

$username = get_string($_POST, "username");
$password = get_string($_POST, "password");

(you can do simliar for get_number, get_boolean, get_array, etc)

To prepare the SQL query to avoid SQL injections, do:

$sql = sprintf(
    "SELECT id FROM user WHERE username = '%s' AND password = MD5('%s')",
        mysql_real_escape_string($user),
        mysql_real_escape_string($password)
); 

To display user-controlled input to avoid XSS, do:

echo htmlspecialchars($data, ENT_QUOTES, 'UTF-8');

It depends on where and how you want to use the user data. You need to know the context you want to insert your data in and the meta characters of that context.

If you just want to allow the user to put text up on your website, htmlspecialchars suffices to escape the HTML meta characters. But if you want to allow certain HTML or want to embed user data in existing HTML elements (like a URL into a A/IMG element), htmlspecialchars is not enough as you’re not in the HTML context anymore but in the URL context.

So entering <script>alert("xss")</script> into a image URL field will yield:

<img src="&lt;script&gt;alert(&quot;xss&quot;)&lt;/script&gt" />

But entering javascript:alert("xss") will succeed:

<img src="javascript:alert(&quot;xss&quot;)" />

Here you should take a look at the fabulous XSS (Cross Site Scripting) Cheat Sheet to see what contexts your user data can be injected in.

strip_tags is not necessary. In most cases strip_tags is just irritating, because some of your users may want to use < and > in their texts. Just use htmlspecialchars (or htmlentities if you prefer) before you echo the texts to the browser.

(Don't forget mysql_real_esacpe_string before you insert anything into your database!)