mysql - Undefined index error in php although i defined it.why? -

i new php please guide , don't make duplicate because can't solution previous solutions.my php code given below

 <?php  if($_server['request_method']=='get'){  $city  = $_get['city'];  $town  = $_get['town']; //$skills=$_post['skills']; require_once('dbconnect.php');   //creating sql query  $sql = "select employees city='".$city."' , town='".$town."'";   //getting result   $r = mysqli_query($con,$sql);   //creating blank array   $result = array();   //looping through records fetched  while($row = mysqli_fetch_array($r)){   //pushing name , id in blank array created   array_push($result,array(  "name"=>$row['name'],  "phone"=>$row['phone'],  "skills"=>$row['skills']  ));  }   //displaying array in json format   echo json_encode(array('result'=>$result));   mysqli_close($con);  }  ?> 

error

notice: undefined index: city in c:\xampp\htdocs\getservices\employeesinfo.php on line 3

notice: undefined index: town in c:\xampp\htdocs\getservices\employeesinfo.php on line 4

warning: mysqli_fetch_array() expects parameter 1 mysqli_result, boolean given in c:\xampp\htdocs\getservices\employeesinfo.php on line 17 {"result":[]}

you need select 1 or more columns, example doing select all select * from.., query this

$sql = "select * employees city='".$city."' , town='".$town."'"; 

update_code:

<?php  if($_server['request_method']=='get' && !empty($_get['city']) && !empty($_get['town'])){   $city  = $_get['city'];  $town  = $_get['town'];  //$skills=$_post['skills']; require_once('dbconnect.php');    //creating sql query   $sql = "select * employees city='".$city."' , town='".$town."'";   //getting result   $r = mysqli_query($con,$sql);   //creating blank array   $result = array();   //looping through records fetched  while($row = mysqli_fetch_array($r)){   //pushing name , id in blank array created   array_push($result,array(  "name"=>$row['name'],  "phone"=>$row['phone'],  "skills"=>$row['skills']  ));  }   //displaying array in json format   echo json_encode(array('result'=>$result));   mysqli_close($con);  }  else{   $message = " fill details first";   }  if(isset($message) && !empty($message) ){  echo "$message"; }  ?>