Call two class object failed

Hi my friend,i wrote two classed on is user,the other is db .The user
have a property which from the db class object .


When i writing my application have both class object invoked at the
same time ,just like
$user_a = new user($para);
$db_a = new db()

Then the problem occurs only one of the class object could work, or
$user_a or $db_a not both,What 's problem of my code

The following is my source code:

class db{
	/**
	 * Constructor
	 */

        var $result;
	var $conn;

	function  db(){
		require_once BOC_BASE_DIR.'/config/db.config.inc';
		$this->conn = mysql_connect($db_host,$db_user,$db_pass);
		//$this->conn = @mysql_connect($db_host,$db_user,$db_pass);
		if (!$this->conn) {
			$this->db_print_error("DB connect failed");
		}

		if (!mysql_select_db($db_name,$this->conn)) {
			$this->db_print_error("DB select failed");
		}
	}

class user{
	private  	$u_sn;
	private  	$u_id;
	private  	$u_name;
	private  	$u_sex;
	private  	$u_image;

        //more property

	private 	$u_info;


	/**
	 * Constructor
	 */
	function __construct($name){
		$sql = "select * from boc_user where u_name = '$name'";
		$u_query = new db();
		$this->u_info = array();
		$u_query->db_query($sql);
		$this->u_info =$u_query->db_fetch_row();

		$this->u_sn = $this->u_info['u_sn'];
		$this->u_id = $this->u_info['u_id'];
		$this->u_name = $this->u_info['u_name'];
		$this->u_sex = $this->u_info['u_sex'];
	
	       //more are give the property value

	}


The two class works well in the single application when only of them
are invoked ,i cost me much time to deal it
Any advice and suggestions are thankful !


All you best
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