$("#country").change(function(){
    $('#city').find('option').remove().end(); //clear the city ddl
    var country = $(this).find("option:selected").text();
    alert(country);
    //do the ajax call
    $.ajax({
        url:'getCity.php'
        type:'GET',
        data:{city:country},
        dataType:'json',
        cache:false,
        success:function(data){

        data=JSON.parse(data); //no need if dataType is set to json
         var ddl = document.getElementById('city');

         for(var c=0;c<obj.length;c++)
              {              
               var option = document.createElement('option');
               option.value = obj[c];
               option.text  = obj[c];                           
               ddl.appendChild(option);
              }


    },
        error:function(jxhr){
        alert(jxhr.responseText);

    }
    });

});

在您的getCity.php中

$country = $_GET['city'];

//do the db query here

$query  = "your query";
$result = mysql_query($query);
$temp = array();
while ($row = mysql_fetch_assoc($result)) {

 if(empty($temp))
 {
   $temp=array($row['city']);
 }
 else
 {  
   array_push($temp,$row['city']);
 }

}
echo (json_encode($temp));