Query construction question

I’m reading sitepoint’s PHP anthology book and they’re discussing PDO. At one point they discuss the prepare and execute methods. Here’s the code:

// make the DSN
$dsn = 'mysql:host=localhost;dbname=world;';
$user = 'user';
$password = 'secret';

$country = 'USA';
// try to make the connection to the database
  $dbh = new PDO($dsn, $user, $password);
  $sql = 'Select * from city where CountryCode =:country';
  $stmt = $dbh->prepare($sql);
  $stmt->bindParam(':country', $country, PDO::PARAM_STR);
  while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
    print $row['Name'] . "\	";
    print $row['CountryCode'] . "\	";
    print $row['Population'] . "\
// if there is a problem we can handle it here
catch (PDOException $e)
  echo 'PDO Exception Caught.  ';
  echo 'Error with the database: <br />';
  echo 'SQL Query: ', $sql;
  echo 'Error: ' . $e->getMessage();

Most of it makes sense however there are two lines I don’t quite get.

  $sql = 'Select * from city where CountryCode =:country';


$stmt->bindParam(':country', $country, PDO::PARAM_STR);

In both of these lines the variable “country” is prefixed with a colon, for example “:country”. What does the colon represent and how does it affect the statement?


Ok well this is a classic case of RTFM. I’m basically jumping ahead on the book and I think that’s what confused me.

So I understand that :country is a placeholder and the real value is added when PDO execute is called. So here’s my next question, why would you bind a parameter this way? Is it so you can reuse the query statement multiple times and simply change the parameter?