Ajax无法向php发送数据使用GET

I create a html, and after I submit, the ajax should send a message to php, but my php doesn't get this data, please tell me how do I do, thank you.

function SendData(){
    $.ajax({
        type: "GET",
        url: "client.php?m=submit",
        success: function(data){
            alert("success");
        },
        error: function(data){
            alert("error");
        }
    });
}

php code always nothing, I know the m isn't exist, but I don't understand.

$message = isset($_GET['m']);
echo $message;

The problem is that you get a boolean with isset();

Do like this:

if(isset($_GET['m'])) {
  $message = $_GET['m'];
  echo $message;
}

you can try like

function SendData(){
    $.ajax({
        type: "GET", //send it through get method
        url: "client.php",
        data: { 
            m: submit   // Here you can send multiple parameters 
         },
        success: function(data){
            alert("success");
        },
        error: function(data){
            alert("error");
        }
    });
}

And you can get the data by

 if(isset($_GET['m'])){echo $_GET['m'];} //gives submit

Change your code to

$message = isset($_GET['m']) ? $_GET['m'] : null;
echo $message;

the php isset() always determine if a variable is set and is not NULL. It will return FALSE if testing a variable that has been set to NULL. You can't echo the result outputted by isset directly using echo, use like this

$message= isset($_GET['m']) ? $_GET['m'] : ' ';
echo $message;

check the documention of isset here http://php.net/manual/en/function.isset.php