How to prevent SQL injections

Use prepared statements and parameterized
queries. These are SQL statements that are sent
to and parsed by the database server separately
from any parameters. This way it is impossible
for an attacker to inject malicious SQL.
You basically have two options to achieve this:
Using PDO:
$stmt = $pdo->prepare('SELECT * FROM
employees WHERE name = :name');
$stmt->execute(array('name' => $name));
foreach ($stmt as $row) {
// do something with $row
}
Using MySQLi:
$stmt = $dbConnection->prepare('SELECT *
FROM employees WHERE name = ?');
$stmt->bind_param('s', $name);
$stmt->execute();
$result = $stmt->get_result();
while ($row = $result->fetch_assoc()) {
// do something with $row
}
PDO
Note that when using PDO to access a MySQL
database real prepared statements are not used
by default. To fix this you have to disable the
emulation of prepared statements. An example
of creating a connection using PDO is:
$dbConnection = new PDO('mysql:dbna
me=dbtest;host=127.0.0.1;charset=utf8', 'user',
'pass');
$dbConnection->setAttribute(PD
O::ATTR_EMULATE_PREPARES, false);
$dbConnection->setAttribute(PD
O::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
In the above example the error mode isn't
strictly necessary, but it is advised to add it.
This way the script will not stop with a Fatal
Error when something goes wrong. And it gives
the developer the chance to catch any error(s)
which are thrown as PDOExceptions.
What is mandatory however is the first
setAttribute() line, which tells PDO to disable
emulated prepared statements and use real
prepared statements. This makes sure the
statement and the values aren't parsed by PHP
before sending it to the MySQL server (giving a
possible attacker no chance to inject malicious
SQL).
Although you can set the charset in the options
of the constructor, it's important to note that
'older' versions of PHP (< 5.3.6) silently ignored
the charset parameter in the DSN.
Explanation
What happens is that the SQL statement you
pass to prepare is parsed and compiled by the
database server. By specifying parameters
(either a ? or a named parameter like :name in
the example above) you tell the database engine
where you want to filter on. Then when you call
execute, the prepared statement is combined
with the parameter values you specify.
The important thing here is that the parameter
values are combined with the compiled
statement, not an SQL string. SQL injection
works by tricking the script into including
malicious strings when it creates SQL to send to
the database. So by sending the actual SQL
separately from the parameters, you limit the
risk of ending up with something you didn't
intend. Any parameters you send when using a
prepared statement will just be treated as
strings (although the database engine may do
some optimization so parameters may end up as
numbers too, of course). In the example above,
if the $name variable contains 'Sarah'; DELETE
FROM employees the result would simply be a
search for the string "'Sarah'; DELETE FROM
employees", and you will not end up with an
empty table.
Another benefit with using prepared statements
is that if you execute the same statement many
times in the same session it will only be parsed
and compiled once, giving you some speed
gains.
Oh, and since you asked about how to do it for
an insert, here's an example (using PDO):
$preparedStatement = $db->prepare('INSERT
INTO table (column) VALUES (:column)');
$preparedStatement->execute(array('column' =>
$unsafeValue));
127.0.0.1
127.0.0.1