$URL值未添加到mySQL数据库PHP中

这是我的代码：

<?php
/**
 * Class to handle articles
 */
class Article
{
// Properties
/**
* @var int The article ID from the database
*/
public $id = null;
/**
* @var int When the article is to be / was first published
  */
public $publicationDate = null;
/**
* @var string Full title of the article
*/
public $title = null;
/**
* @var string A short summary of the article
*/
public $summary = null;
/**
* @var string The HTML content of the article
*/
 public $content = null;

/**
* Sets the object's properties using the values in the supplied array
*
* @param assoc The property values
*/
public function __construct( $data=array() ) {
if ( isset( $data['id'] ) ) $this->id = (int) $data['id'];
if ( isset( $data['publicationDate'] ) ) $this->publicationDate = (int) $data['publicationDate'];
if ( isset( $data['title'] ) ) $this->title = preg_replace ( "/[^'.','-'_'''"'@'?'!':'$ a-zA-Z0-9()]/", "", $data['title'] );
if ( isset( $data['summary'] ) ) $this->summary = preg_replace ( "/[^'.','-'_'''"'@'?'!':'$ a-zA-Z0-9()]/", "", $data['summary'] );
if ( isset( $data['content'] ) ) $this->content = $data['content'];
}

/**
* Sets the object's properties using the edit form post values in the supplied array
*
* @param assoc The form post values
*/
public function storeFormValues ( $params ) {
// Store all the parameters
$this->__construct( $params );
// Parse and store the publication date
if ( isset($params['publicationDate']) ) {
  $publicationDate = explode ( '-', $params['publicationDate'] );
  if ( count($publicationDate) == 3 ) {
    list ( $y, $m, $d ) = $publicationDate;
    $this->publicationDate;
  }
 }
}

/**
 * Returns an Article object matching the given article ID
*
* @param int The article ID
* @return Article|false The article object, or false if the record was not found or there was a problem
*/
public static function getById( $id ) {
$conn = new PDO( DB_DSN, DB_USERNAME, DB_PASSWORD );
$sql = "SELECT *, UNIX_TIMESTAMP(publicationDate) AS publicationDate FROM articles WHERE id = :id";
$st = $conn->prepare( $sql );
$st->bindValue( ":id", $id, PDO::PARAM_INT );
$st->execute();
$row = $st->fetch();
$conn = null;
if ( $row ) return new Article( $row );
}

/**
* Returns all (or a range of) Article objects in the DB
*
* @param int Optional The number of rows to return (default=all)
* @param string Optional column by which to order the articles (default="publicationDate ASC, id ASC")
* @return Array|false A two-element array : results => array, a list of Article objects; totalRows => Total number of articles
*/
public static function getList( $numRows=1000000, $order="publicationDate DESC, ID DESC" ) {
$conn = new PDO( DB_DSN, DB_USERNAME, DB_PASSWORD );
$sql = "SELECT SQL_CALC_FOUND_ROWS *, UNIX_TIMESTAMP(publicationDate) AS publicationDate FROM articles
        ORDER BY " . mysql_escape_string($order) . " LIMIT :numRows";
$st = $conn->prepare( $sql );
$st->bindValue( ":numRows", $numRows, PDO::PARAM_INT );
$st->execute();
$list = array();
while ( $row = $st->fetch() ) {
  $article = new Article( $row );
  $list[] = $article;
}
// Now get the total number of articles that matched the criteria
$sql = "SELECT FOUND_ROWS() AS totalRows";
$totalRows = $conn->query( $sql )->fetch();
$conn = null;
return ( array ( "results" => $list, "totalRows" => $totalRows[0] ) );
}

/**
* Inserts the current Article object into the database, and sets its ID property.
*/
public function insert() {
// Does the Article object already have an ID?
if ( !is_null( $this->id ) ) trigger_error ( "Article::insert(): Attempt to insert an Article object that already has its ID property set (to $this->id).", E_USER_ERROR );
$preURL = $data['title'];
$preURL2 = preg_replace('/[^a-z0-9]/i',' ', $preURL);
$preURL3 = str_replace(" ","-", $preURL2);
$url = $date.'/'.$preURL3.'.html';
// Insert the Article
$conn = new PDO( DB_DSN, DB_USERNAME, DB_PASSWORD );
$sql = "INSERT INTO articles ( publicationDate, title, summary, content, url) 
       VALUES ( FROM_UNIXTIME(:publicationDate), :title, :summary, :content, :url )";
$st = $conn->prepare ( $sql );
$st->bindValue( ":publicationDate", $this->publicationDate, PDO::PARAM_INT );
$st->bindValue( ":title", $this->title, PDO::PARAM_STR );
$st->bindValue( ":summary", $this->summary, PDO::PARAM_STR );
$st->bindValue( ":content", $this->content, PDO::PARAM_STR );
$st->bindValue( ":url", $this->url, PDO::PARAM_STR ); 
$st->execute();
$this->id = $conn->lastInsertId();
$conn = null;
}
?>

这并没有按照我的意愿将'titl'e的精简版本添加到Url行。我做错了什么吗？我是否在某个地方错误地定义了一个变量？我没有得到任何错误-只是在url行中留下一个null值。

谢谢。