PHP text box limit using javascript

<script language="javascript" type="text/javascript">
function limitText(limitField, limitCount, limitNum) {
    if (limitField.value.length > limitNum) {
        limitField.value = limitField.value.substring(0, limitNum);
    } else {
        limitCount.value = limitNum - limitField.value.length;
    }
}
</script>

<form name="myform">
<input name="limitedtextfield" type="text" onKeyDown="limitText(this.form.limitedtextfield,this.form.countdown,15);"
onKeyUp="limitText(this.form.limitedtextfield,this.form.countdown,15);" maxlength="15"><br>
<font size="1">(Maximum characters: 15)<br>
You have <input readonly type="text" name="countdown" size="3" value="15"> characters left.</font>
</form>