$query = "SELECT username from my_table where username='bob'";
$result = MysqL_query($query);

if(MysqL_num_rows($result) > 0)
{
    // row exists. do whatever you would like to do.
}

如果要使用PDO (PHP Data Object),as alex建议,那么使用以下代码 –

$dbh = new PDO("MysqL:host=your_host_name;dbname=your_db_name",$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.
}