4. Never run unescaped queries

PHP has a feature, enabled by default, that automatically escapes (adds a backslash in front of) certain characters that come in from a GET, POST, or COOKIE. The single quote (') is one example of a character that is escaped automatically. This is done so that if you include input variables in your SQL queries, it will not treat single quotes as part of the query. Say your user entered $name from a form and you performed this query:

UPDATE users SET Name='$name' WHERE ID=1;

Normally, if they had entered $name with single quotes in them, they would be escaped, so MySQL would see this:

UPDATE users SET Name='Joe\'s' WHERE ID=1

so that the single quote entered into "Joe's" would not interfere with the query syntax.

In some situations, you may use stripslashes() on an input variable. If you put the variable into a query, make sure to use addslashes() or mysql_escape_string() to escape the single quotes before your run the query. Imagine if an unslashed query went in, and a malicious user had entered part of a query as their name!

UPDATE users SET Name='Joe',Admin='1' WHERE ID=1

On the input form, the user would have entered:

Joe',Admin='1

As their name, and since the single quotes were not escaped, he or she would be able to actually end the name definition, place in a comma, and set another variable called Admin!

The final query with input in blue would look like this:

UPDATE users SET Name='Joe',Admin='1' WHERE ID=1

In some configurations, magic_quotes_gpc (the feature that automatically adds slashes to all input) is actually set to OFF. You can use the function get_magic_quotes_gpc() to see if it's on or not (it returns true or false). If it returns false, simply use addslashes() to add slashes to all of the input (it is easiest if you use $_POST, $_GET, and $_COOKIE or $HTTP_POST_VARS, $HTTP_GET_VARS, and $HTTP_COOKIE_VARS, instead of globals because you could step through those arrays using a foreach() loop and add slashes to each one).