注册验证一直失败

<?php     require_once 'db.php';      session_start();          $name =  $_POST["name"] ;     $mail = $_POST["email"] ;	 $pwd=  $_POST["key"] ;	 $pwd2 =  $_POST["rekey"] ;	 $agreement =  $_POST["agreement"] ;     	 	 if($agreement=="T"){	 	if($pwd==$pwd2){	 		$sql="select * from user";		    $result = $db->query($sql);		 		    if($result && $row=$result->fetch_assoc()){		    	if($name==$row["name"]){			 	 	echo '<script>';			        echo "alert('用户已存在');";					echo "window.location='enroll.html'";			        echo "</script>";					exit;			    }elseif($mail==$row["mail"]){			 	 	echo '<script>';			        echo "alert('该邮箱已使用');";					echo "window.location='enroll.html'";			        echo "</script>";					exit;			 	}else{			 		$sql="insert into user(name,represent,pwd,mail,time) value('{$name}',0,'{$pwd}','{$mail}',now())";					$result = $db->query($sql);					if($result && $db->affected_rows>0){						$sql="select * from user where name='{$name}'";						$result = $db->query($sql);						if($result && $row=$result->fetch_assoc()){							$uid=$row['uid'];						}					}								$result->free_result;		            $db->close;											$sql="insert into userphoto(uid,name,type,url,size,time) value('{$uid}','steve','.png','./img/userphoto/steve.png','6.24kb',now())";				    $result = $db->query($sql);		            if($result && $db->affected_rows>0){				     	echo '<script>';				        echo "alert('注册成功');";				        echo "window.location='index.php'";				        echo "</script>";				        exit;				    }else{				        echo '<script>';				        echo "alert('注册失败');";				        echo "window.location='enroll.html'";				        echo "</script>";						exit;				    }			 	}		    }			$result->free_result;            $db->close;		 		 	}else{	 		echo '<script>';	        echo "alert('密码不一致，请重新输入');";	        echo "window.location='enroll.html'";	        echo "</script>";			exit;	 	}	 }else{	 	echo '<script>';        echo "alert('未同意协议，注册失败');";		echo "window.location='enroll.html'";        echo "</script>";		exit;	 }        $result->free_result;     $db->close;?>