如何在AJAX中调用php函数？

Question

我有一个调用php函数的表单，并在按钮被提交时返回转换后的货币。我需要在ajax中调用这个php函数并返回结果（结果显示在textfield结果中）而不按提交按钮。我希望当用户输入他想要转换的金额时，无需按下提交按钮即可自动调用该功能。下面是我的表单和php函数：

  <form class="form-signin" action="" method="post">

        <h4 class="form-signin-heading"style="color:white;">Check Our Rates Here!</h4>
        <Strong style="color:white;">i have:</strong>
            <select class="form-control" name="from"id="from">
            <option value="<?php echo getcurrency();?>"><?php echo getcurrency();?></option>
            <option value="USD">American Dollar(USD)</option>
            <option value="GBP">Britsh Pound(GBP)</option>


        </select>
        <input type="text" name="amount"id="amount"class="form-control" Amount="Your email"required autofocus>
        <Strong style="color:white;">Reciver Gets:</strong>
           <select class="form-control" name="to"id="to" >
            <option value="USD">American Dollar(USD)</option>
            <option value="GBP">Britsh Pound(GBP)</option>
            <option value="UGX">uganda shillings (UGX)</option>
            <option value="KES">Kenya Shilling(KES)</option>
            <option value="EUR">Euro(EUR)</option>

        </select>
        <input class="form-control" id="results"name="results"value="<?php if(isset($_POST['amount'],$_POST['from'],$_POST['to'])){
            $amount = $_POST['amount'];
             $from = $_POST['from'];
             $to = $_POST['to'];

         $currency = currencyConverter($from,$to,$amount);
            echo $currency;


        }?>">
        <button class="btn btn-lg btn-primary btn-block" name="submit"type="submit">Convert</button>

    </form>

这是我的php货币转换功能           

function currencyConverter($from,$to,$amount){
    $yql_base_url = "http://query.yahooapis.com/v1/public/yql";
    $yql_query = 'select * from yahoo.finance.xchange where pair in ("'.$from.$to.'")';
    $yql_query_url = $yql_base_url . "?q=" . urlencode($yql_query);
    $yql_query_url .= "&format=json&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys";
    $yql_session = file_get_contents($yql_query_url);
    $yql_json =  json_decode($yql_session,true);
    $currency_output = (float) $amount*$yql_json['query']['results']['rate']['Rate'];
    if($from=='AED'&&$to=='USD'){
        $aedtousd='4';
        $finalaedtousd = $currency_output-$aedtousd;
        return $finalaedtousd;  

    }else if($from=='USD'&&$to=='AED') {
        $aedtousd ='0.033';
        return $currency_output-$aedtousd;
    }else if($from=='AED'&&$to=='UGX'){
        $eadtougx ='18';
        return $currency_output-$eadtougx;
    }else{
        return $currency_output;
    }

}

?>