How to call class's method using jquery Ajax call

Try this:

hello.php

class hello
{
      public function test()
      {
        //some data
      }

      public function abc()
      {
        //some data
      }
}
if(isset($_GET['method'])){
   $hello = new hello;
   $hello->$_GET['method']();
}

b.php

 $.ajax({
    url : 'hello.php?method=test', //just for example i have written it bcz it should call only test() not abc()..
   })

By they way, it's not secure to expose your class via ajax requests.

One approach is to pass the name of the class, the constructor arguments, and the method name and arguments etc. via ajax POST or GET, for example:

var url = 'callMethod.php';
var data = {
    str_className: 'Hello',
    arr_consArgs: {arg1: 'test1'},
    str_methodName: 'test'
};
$.post(url, data, function(response) {
    etc.
});

In PHP script named callMethod.php:

/* Place your 'Hello' class here */

// class
$str_className = !empty($_POST["str_className"]) ? $_POST["str_className"] : NULL;
if ($str_className) {
    // constructor
    $arr_consArgs = !empty($_POST["arr_consArgs"]) ? $_POST["arr_consArgs"] : array();

    // method
    $str_methodName = !empty($_POST["str_methodName"]) ? $_POST["str_methodName"] : NULL;
    if (!empty($str_methodName)) {
        $arr_methodArgs = !empty($_POST["arr_methodArgs"]) ? $_POST["arr_methodArgs"] : array();
    }

    // call constructor
    $obj = fuNew($str_className, $arr_consArgs);

    // call method
    $output = NULL;
    if (!empty($str_methodName)) 
        $output .= call_user_func_array(array($obj, $str_methodName), $arr_methodArgs);

    // echo output
    echo $output;

}

where:

function fuNew($classNameOrObj, $arr_constructionParams = array()) {
    $class = new ReflectionClass($classNameOrObj);
    if (empty($arr_constructionParams))
        return $class->newInstance();
    return $class->newInstanceArgs($arr_constructionParams);
}