check if SQL row exists with PHP

I'm using MySQL with PHP and I need to do something like this (pseudocode):

if (s开发者_JAVA百科ql row exists where username='bob')
{
    // do this stuff
}

If you would like to use PDO (PHP Data Object), then use the following code:

$dbh = new PDO("mysql:host=your_host_name;dbname=your_db_name", $user, $pass);
$stmt = $dbh->prepare("SELECT username from my_table where username = ':name'");
$stmt->bindParam(":name", "bob");
$stmt->execute();

if($stmt->rowCount() > 0)
{
    // row exists. do whatever you want to do.
}

Sayem's answer has the most upvotes, but I believe it is incorrect regarding PDO.

From the PHP docs:

For most databases, PDOStatement::rowCount() does not return the number of rows affected by a SELECT statement. Instead, use PDO::query() to issue a SELECT COUNT(*) statement with the same predicates as your intended SELECT statement, then use PDOStatement::fetchColumn() to retrieve the number of rows that will be returned.

$sql = "SELECT COUNT(*) FROM fruit WHERE calories > 100";
if ($res = $conn->query($sql)) {

  /* Check the number of rows that match the SELECT statement */
  if ($res->fetchColumn() > 0) {

       /* Issue the real SELECT statement and work with the results */
       $sql = "SELECT name FROM fruit WHERE calories > 100";
       foreach ($conn->query($sql) as $row) {
           print "Name: " .  $row['NAME'] . "\n";
       }
  }
  /* No rows matched -- do something else */
  else {
      print "No rows matched the query.";
  }
}

$res = null;
$conn = null;

another approach

$user = "bob";
$user = mysql_real_escape_string($user);
$result = mysql_query("SELECT COUNT(*) AS num_rows FROM my_table WHERE username='{$user}' LIMIT 1;");
$row = mysql_fetch_array($result);
if($row["num_rows"] > 0){
   //user exists
}