if(isset($_SESSION['login'])){
   $logins=$_SESSION['login'];
   echo $logins->getId();(linea 33)
}

public function getId(){
   return $this->id;
}

include("conexionBD.php");
.
.
.
header("index.php");
exit;
 

class BaseDatos{
	var $hostname,$user,$password,$dtb;
 
    function BaseDatos(){
		if($_SERVER['REMOTE_ADDR']=="127.0.0.1"){
			$hostname="localhost";
			$user="root";
			$password="";
			$dtb="colegio";
		}else{
			$hostname="localhost";
			$user="***";
			$password="**";
			$dtb="colegio";
		}	
        $this->hostname = $hostname;
		$this->user = $user;
		$this->password = $password;
		$this->dtb=$dtb;
    }
 
    function conect(){
		$isOk=true;
		$conexion = mysql_connect($this->hostname,$this->user,$this->password);
		if(!$conexion){
			$isOk=false;
		}else{
			$c=mysql_select_db($this->dtb, $conexion);
			if(!$c) $isOk=false;
		}
		return $isOk;
    }
}
 
 

require("includes/classes/BaseDatos.php");
$bd=new BaseDatos();
$bd->conect();
.
.
.
header("index.php");
 

<?php
if(isset($_POST['login'])){
	$redir=$_SERVER['HTTP_REFERER'];
	$url=explode("?",$redir);
	$redir=$url[0];
 
	$user=$_POST['user'];
	$pass=$_POST['pass'];
	if($user==""){
		header ("Location: $redir?error_login=0");
		exit;
	}
	if($pass==""){
		header ("Location: $redir?error_login=0");
		exit;
	}
	/** comprobem que el usuario y contraseña estan a la BD*/
	include("../conexion.php");
	$sql="SELECT `id`,`pass`,`carrec`,`ban`,`last_day`,`entradas`,`alta` FROM usuari";
	$sql .=" WHERE `nick` LIKE '".$user."' LIMIT 1";
	$result=mysql_query($sql);
	if(mysql_num_rows($result)==0){
		header ("Location: $redir?error_login=1");
		exit;
	}
	$row=mysql_fetch_array($result);
	if($row[2]==1){//comprobem k lusuari no estigui ban 1-> BAN; 0-> no ban
		header ("Location: $redir?error_login=2");
		exit;
	}
	/*comprobem k estigui de alta*/
	if($row['alta']!=1){
		header ("Location: $redir?error_login=4");
		exit;
	}
	/** comprovem si la contraseña coincideix*/
	if($pass==$row['pass']){
		$data=date("d/m/Y");
		//actualitzem al ultima visita
$sql="UPDATE `usuari` SET `last_day` = '".$data."',`entradas`=`entradas`+1, `ip`='".$_SERVER['REMOTE_ADDR']."' WHERE `nick` = '".$user."' LIMIT 1" ;
		mysql_query($sql);
		/*--> linea 42*/session_start();
		$_SESSION['id']=$row['id'];
		$_SESSION['user']=$user;
		$_SESSION['pass']=$pass;
		$_SESSION['carrec']=$row['carrec'];
		$_SESSION['entradas']=$row['entradas'];
		$url="../index.php?".session_name()."=".session_id();
		/*--> linea 49*/header ("Location: $url");
	}else{
		header ("Location: $redir?error_login=3");
		exit;
	}
	mysql_free_result($result);
	mysql_close();
}else{
	header("Location: login.php");
}
?>
 

CREATE TABLE USUARI(
 
	id INT(2) PRIMARY KEY AUTO_INCREMENT ,
 
	nick VARCHAR(25) NOT NULL ,
 
	pass VARCHAR(25) NOT NULL ,
 
	email VARCHAR(25) NOT NULL ,
 
	edad INT(4),
 
	sexo CHAR(1),
 
	data_creacio DATE DEFAULT 'NOW()',
 
	carrec CHAR(1) DEFAULT 'U',
 
	ban INT(1) DEFAULT '0',
 
	last_time TIME DEFAULT 'CURTIME()',
 
	last_day VARCHAR(10),
 
	entradas INT(4) DEFAULT 0,
 
	comentari VARCHAR(100),
 
	ip VARCHAR(15),
 
	alta INT(1) DEFAULT 0,
 
	codigo_alta INT(5)
 
) AUTO_INCREMENT = 1;

CREATE TABLE usuaris(
	id INT(3) PRIMARY KEY,
	usuari VARCHAR(50) UNIQUE,
	pass TEXT,
	carrec VARCHAR(15) DEFAULT 'alumne',
	FOREIGN KEY(id) REFERENCES alumne(id)
);
CREATE TABLE alumne(
	id INT(3) PRIMARY KEY AUTO_INCREMENT,
	nom VARCHAR(50),
	cognom1 VARCHAR(50),
	cognom2 VARCHAR(50),
	curs INT(1),
) AUTO_INCREMENT = 1;

$sql="SELECT `alumne.id`,`alumne.nom`,`alumne.cognom1`,`alumne.cognom2`,`usuaris.pass` FROM alumne,usuaris";
$sql .=" WHERE `alumne.id`=`usuaris.id` AND `usuaris.usuari` LIKE '".$user."' LIMIT 1";

 consulta SQL : 

SELECT `alumne.id` , `alumne.nom` , `alumne.cognom1` , `alumne.cognom2` , `usuaris.pass`
FROM alumne, usuaris
WHERE `alumne.id` = `usuaris.id` AND `usuaris.usuari`
LIKE 'mtrepat'
LIMIT 1

MySQL ha dicho:

#1054 - La columna 'alumne.id' en field list es desconocida

mysql_query ("INSERT INTO `user`(`nom`)VALUES(''$usuario'')",$conexion);

mysql_query ("INSERT INTO user(nom)VALUES(''$usuario'')",$conexion);