How to pass value from dropdown list and show in another form

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PHP

i have table currency, in it have column currency (USD) and column ExchRate (100.00)
in my dropdown list. i want to show currency value. after i click on the dropdown list (using 'onchange'), the result of ExchRate will be display in readonly form below the dropdown. i saw some tutorial using ajax and function, but i dont really understand how it catch and fetch the data.

What I have tried:

<select id="exchangerate" name='exchangerate' onchange="initExchRate()">
							<option value="">-</option>
							<?	//query for dropdown currency
								$stroption = "SELECT * FROM Currency";
								$rstoption = mysql_query($stroption);
								
								while($objoption = mysql_fetch_object($rstoption)){
									echo "<option value='".$objoption->ExchRate."'>".$objoption->Currency."</option>";
								}
							?>
						</select>
						<input name="txtRate1" type="text" id="txtRate1" size="15" maxlength="20" class="txb" tabindex="<?=$tabindex++?>"readonly>

function initExchRate(){
	var exchangerate = $("#exchangerate").val();
	var rate = document.getElementById('txtRate1');
	var options = new Object();

	var ajaxy = $.ajax({
		method: "GET",
		url: "../../../script/ajaxphp3.php",
		dataType: "JSON",
		data: {
			getCase: "getExchangerate",
			Exchangerate: exchangerate
		}
	})
	.done(function(data){
		var exchrate = data.ExchRate;

		getExchRate();
	});
}

function getExchRate(){
	var exchangerate = $("#exchangerate").val();
	var excRate = $("#txtRate1").val();

	var ajaxy = $.ajax({
		method: "GET",
		url: "../../../script/ajaxphp3.php",
					dataType: "JSON",
					data: { 
						getCase: "getExcRate",
						Exchangerate: exchangerate
						ExcRate: excRate
					}
				})
				.done(function(data){
					setExcRateFields(data);
				});
}
function setExcRateFields(Currency){

	document.getElementById("txtRate1").value = Currency.ExchRate;
}