How to set up qiwi api?

Question

Qiwi

there is an old script of 2012!
Interested in payment, everything is already in the script and the handler and that's it!
i91891bd.beget.tech/catalog/kupit-serebro-R2-Onlin... here you can make sure that the payment is working!
I didn’t understand where to edit qiwi api
Everything works fine on the site, after payment it transfers to qiwi billing!

elseif($kurs=="QIWI"){ ?>
    <script type="text/javascript">
var ie = document.all;
var moz = (navigator.userAgent.indexOf("Mozilla") != -1);
var opera = window.opera;
var brodilka = "";
if(ie && !opera){brodilka = "ie";}
else if(moz){brodilka = "moz";}
else if(opera){brodilka = "opera";}
var inputMasks = new Array();

function kdown(inpt, ev){
    var id = inpt.getAttribute("id");
    var idS = id.substring(0, id.length - 1);
    var idN = Number(id.substring(id.length - 1));
    inputMasks[idS].BlKPress(idN, inpt, ev);
}

function kup(inpt, ck){
    if(Number(inpt.getAttribute("size")) == inpt.value.length){
        var id = inpt.getAttribute("id");
        var idS = id.substring(0, id.length - 1);
        var idN = Number((id.substring(id.length - 1))) + 1;
        var t = document.getElementById(idS + idN);
        if(ck!=8 && ck!=9){
            if(t){t.focus();}
        } else if (ck==8) {
            inpt.value = inpt.value.substring(0, inpt.value.length - 1);
        }
    }
}

function Mask(fieldObj){
    var template = "(\\d{3})\\d{3}-\\d{2}-\\d{2}";
    var parts = [];
    var blocks = [];
    var order = [];
    var value = "";

    var Block = function(pattern){
        var inptsize = Number(pattern.substring(3, pattern.indexOf('}')));
        var idS = fieldObj.getAttribute("id");
        var idN = blocks.length;
        var text = "";

        var checkKey = function(ck){
            return ((ck >= 48) && (ck <= 57)) || ((ck >= 96) && (ck <= 105)) || (ck == 27) || (ck == 8) || (ck == 9) || (ck == 13) || (ck == 45) || (ck == 46) || (ck == 144) || ((ck >= 33) && (ck <= 40)) || ((ck >= 16) && (ck <= 18)) || ((ck >= 112) && (ck <= 123));
        }

        this.makeInput = function(){
            return "<input type='text' " + "size='" + inptsize + "' maxlength='" + inptsize + "'"  + " id='" + idS + idN + "' onKeyDown='kdown(this, event)' onKeyUp='kup(this, event.keyCode)' value='" + text + "'>";
        }

        this.key = function(inpt, ev){
            if(opera) return;
            if(!checkKey(ev.keyCode)){
                switch(brodilka){
                    case "ie":
                        ev.cancelBubble = true;
                        ev.returnValue = false;
                    break;
                    case "moz":
                        ev.preventDefault();
                        ev.stopPropagation();
                    break;
                    case "opera":
                    break;
                    default:
                }
                return;
            }

            if(ev.keyCode == 8 && inpt.value == ""){
                var tid = inpt.getAttribute("id");
                var tidS = tid.substring(0, tid.length - 1);
                var tidN = Number(tid.substring(tid.length - 1)) - 1;
                var t = document.getElementById(tidS + tidN);
                if(t != null) t.focus();
            }
        }

        this.getText = function(){
            text = document.getElementById(idS + idN).value;
            return text;
        }

        this.setText = function(val){
            text = val;
        }

        this.getSize = function() {
            return inptsize;
        }
    }

    this.drawInputs = function(){
        var inputStr = "<span class='Field'>";
        var p = 0;
        var b = 0;
        for (var i = 0; i < order.length; i++) {
            if (order[i] == "p") {
                inputStr += parts[p];
                p++;
            } else {
                inputStr += blocks[b].makeInput();
                b++;
            }
        }
        inputStr += "</span>";
        document.getElementById("div_" + fieldObj.getAttribute("id")).innerHTML = inputStr;
        fieldObj.style.display = "none";
    }

    this.buildFromFields = function() {// constructor
        var tmpstr = template;
        while(tmpstr.indexOf("\\") != -1){
            var slash = tmpstr.indexOf("\\");
            var d = "";
            if(tmpstr.substring(0, slash) != ""){
                parts[parts.length] = tmpstr.substring(0, slash);
                order[order.length] = 'p';
                tmpstr = tmpstr.substring(slash);
            }
            var q = tmpstr.indexOf('}');
            blocks[blocks.length] = new Block(tmpstr.substring(0, q + 1), d);
            tmpstr = tmpstr.substring(q + 1);
            order[order.length] = 'b';
        }
        if (tmpstr != "") {
            parts[parts.length] = tmpstr;
            order[order.length] = 'p';
        }
        this.drawInputs();
    }

    this.buildFromFields();

    this.BlKPress = function(idN, inpt, ev){
        blocks[idN].key(inpt, ev);
    }

    this.makeHInput = function(){
        var name = fieldObj.getAttribute("name");
        document.getElementById("div_" + fieldObj.getAttribute("id")).innerHTML =
            "<input type='text' readonly='readonly' name='" + name + "' value='" + this.getValue() + "'>";
    }

    this.getFName = function(){
        return fieldObj.getAttribute("name");
    }

    this.getValue = function(){
        value = "";
        var p = 0;
        var b = 0;
        for(var i = 0; i < order.length; i++){
            /*if(order[i] == 'p'){
                value += parts[p];
                p++;
            } else {
                value += blocks[b].getText();
                b++;
            }
            */
        	if (order[i] != 'p') {
        		value += blocks[b].getText();
        		b++;
        	}
        }
        return value;
    }

    this.check = function(){
        for(var i in blocks){
            if (blocks[i].getText().length == 0) return false;
        }
        return true;
    }
}
</script>
<div style="margin-top:33px;margin-left:280px; padding:5px; width:450px; border:1px solid #ddd; background:#fff; border-radius: 7px; -webkit-border-radius: 7px; -moz-border-radius: 7px; font:normal 14px/14px Geneva,Verdana,Arial,Helvetica,Tahoma,sans-serif;text-align:left;">
  <form action="http://w.qiwi.ru/setInetBill.do" method="get" accept-charset="windows-1251" onSubmit="return checkSubmit();">
  
    <input type="hidden" name="from" value="4889"/>
    <input type="hidden" name="lifetime" value="0.0"/>
    <input type="hidden" name="check_agt" value="false"/>
    
    <!-- используйте это поле для передачи уникального идентификатора заказа/платежа в вашей системе -->
    	<!-- <input type="hidden" name="txn_id" value=""/> -->
    	
    <p style="text-align:center; color:#006699; padding:20px 0px; background:url(https://ishop.qiwi.ru/img/button/logo_31x50.jpg) no-repeat 10px 50%;">Оплатить счёт</p>
    <table style="border-collapse:collapse;">
      <tr styackle="bground:#f1f5fa;">
        <td style="color:#a3b52d; width:45%; text-align:center; padding:10px 0px;">Мобильный телефон (пример: 9057772233)</td>
        <td style="padding:10px">
          <input type="text" name="to" id="idto" style="width:130px; border: 1px inset #555;"></input>
          <span id="div_idto"></span>
          <script type="text/javascript">
            inputMasks["idto"] = new Mask(document.getElementById("idto"));
            function checkSubmit() {
              if (inputMasks["idto"].getValue().match(/^\d{10}$/)) {
                document.getElementById("idto").setAttribute("disabled", "disabled");
                inputMasks["idto"].makeHInput();
                return true;
              } else {
                alert("Введите номер телефона в федеральном формате без \"8\" и без \"+7\"");
                return false;
              }
            }
          </script>
    			</td>
      </tr>
      <tr>
        <td style="color:#a3b52d; padding:10px 0px; width:45%; text-align:center;">Сумма</td>
        <td style="padding:10px">
          <input type="text" name="amount_rub" value="<?=$pay;?>" maxlength="5" style="width:50px; text-align:right;  border: 1px inset #555;" /> руб. 
          
        </td>
      </tr>
      <tr style="background:#f1f5fa;">
        <td style="color:#a3b52d; padding:10px 0px; width:45%; text-align:center;">Комментарий</td>
        <td style="padding:10px"><textarea rows="2" cols="30" name="com" style="width:200px; border: 1px inset #555;"></textarea></td>
      </tr>
    </table>
    <p style="text-align:center;"><input type="submit" value="Оплатить счёт за покупку" style=" padding:10px 0;border:none; background:url(https://ishop.qiwi.ru/img/button/superBtBlue.jpg) no-repeat 0 50%; color:#fff; width:300px;"/></p>
  </form>
</div>
<?php }elseif($kurs=="WMR" || $kurs=="WMU" || $kurs=="WMZ" || $kurs=="WME") { ?>
<form method="POST" action="https://merchant.webmoney.ru/lmi/payment.asp">
  <input type="hidden" name="LMI_PAYMENT_AMOUNT" value="<?=$pay;?>">
  <input type="hidden" name="LMI_PAYMENT_DESC" value="Оплата игровой валюты">
  <input type="hidden" name="LMI_PAYMENT_NO" value="1234">
  <input type="hidden" name="LMI_PAYEE_PURSE" value="R301947179023">
  <input type="hidden" name="LMI_SIM_MODE" value="0">
  <input type="hidden" name="LMI_RESULT_URL" value="http://www.shop.com/result.asp">
  <input type="hidden" name="LMI_SUCCESS_URL" value="http://www.shop.com/success.html">
  <input type="hidden" name="LMI_SUCCESS_METHOD" value="2">
  <input type="hidden" name="LMI_FAIL_URL" value="http://www.shop.com/fail.html">
  <input type="hidden" name="LMI_FAIL_METHOD" value="2">
  <input type="submit" />
</form>

<?php } else { ?>
<div style="text-align:center;font-size:14pt;margin-top:30px;">
Ваш заказ принят и будет обработан в ближайшее время! Вы будете проинфомированы об этом по E-mail или ICQ. Спасибо за заказ!<br /><br />
Вы можете перейти в <a href="/catalog.html">наш каталог</a>, чтобы посмотреть другие предоставляемые нами услуги.
</div>
    <?php } ?>
</div><!-- /.sidecenter -->
</div><!-- /.contentblock -->
  
<?php require_once 'inc/footer.inc';?>