Linux, Apache, MySQL, PHP

<?php
@session_start();
header('Content-Type: text/html; charset=ISO-8859-1');
include('includes/config.php');
?>
		<link rel="stylesheet" type="text/css" href="css/chat.css" />
<div id="sidebar_header">Garderobe</div>
<div id="sidebar_content">
	<p style="display: inline;">Skid i havet.</p><br /><br />
	<select name="sko">
<?php

	if(isset($_SESSION['logget_ind']) && $_SESSION['logget_ind'] == true) {
		$getSko = mysql_query("SELECT `navn` FROM `chat_clothes` WHERE `ejer` = '".$_SESSION['brugernavn']."' AND `type` = 'sko'");

		
		while ($showSko = mysql_fetch_array($getSko)) {
			echo '<option id="sko" name="sko" style="width: 300px;">'.$showSko['navn'].'</option><br />'
			;
			}
	}
?>

</div>

			</select>
<?php
	if (isset($_POST['sko'])) {
		

						mysql_query("UPDATE chat_brugere SET shoes='".$getSko."' WHERE id='".$_SESSION['id']."'");

						echo 'sko er opdateret!';
				} else {
					echo '';
				}
			
			echo '
			<form action="nygad.php" method="POST">

				<div>
					<table>
					
						<tr>
							<td><center><input type="submit" name="sko" value="Opdater!" /></center></td>
						</tr>
					</table>
				</div>
			</form>
			';
		?>

Hello
I want the source code for a project
  Solver of third-degree equations php
Hope you can help me

this page has an error when pressing the money button to consult, I have sql code but it is not working properly

sql code used:

SELECT
e.nomeEvento,p.nomecompl,c.nomecasa, COALESCE(l.observacao,'Sem informação') AS `observacao`, CONCAT('R$ ',REPLACE(REPLACE(REPLACE(FORMAT(IFNULL(l.valorExtornado, 0), 2), ".", "@"), ",", "."), "@", ",")) AS `valorExtornado`,
l.idpart, l.parcela, p.id AS idParticipante, DATE_FORMAT(l.datavcto, '%d/%m/%Y') AS `dataVcto`,
CONCAT('R$ ',REPLACE(REPLACE(REPLACE(FORMAT(l.valor, 2), ".", "@"), ",", "."), "@", ",")) AS `valorDevido`,
IF(l.datapgto IS NULL,'danger','success') AS `pagoCSS`,
IF(l.datapgto IS NULL,'Pendente','Pago') AS `pago`,
IFNULL(DATE_FORMAT(l.datapgto,'%d/%m/%Y'),'') AS `dataPgto`, CONCAT('R$ ',REPLACE(REPLACE(REPLACE(FORMAT(IFNULL(l.valorpago, 0), 2), ".", "@"), ",", "."), "@", ",")) AS `valorPago`,
(SELECT COUNT(1) FROM UF_financeiro_lctofinan WHERE id_eventos_participante=p.id  AND fl_excluido='N') AS NUM_PARCELAS
FROM UF_financeiro_lctofinan AS l
LEFT JOIN UF_eventos_participantes AS p ON l.id_eventos_participante=p.id
LEFT JOIN UF_eventos AS e ON e.id_evento=p.id_evento
LEFT JOIN UF_casas AS c ON p.idcasainscr = c.idcasa
LIMIT 150 -- retirar esse LIMIT depois, isso é apenas para debugar

<?php

    if ($_SERVER["REQUEST_METHOD"] == "POST") {

        // access
        $secretKey = '6LdhAK8UAAAAAD3Uaj5ILNiO5EPYPtDr2Wmr3sRO';
        $captcha = $_POST['g-recaptcha-response'];

        if(!$captcha){
          echo '<p class="alert alert-warning">Please check the the captcha form.</p>';
          exit;
        }

        # FIX: Replace this email with recipient email
        $mail_to = "dixie7@xtra.co.nz";
        
        # Sender Data
        $subject = trim($_POST["subject"]);
        $name = str_replace(array("\r","\n"),array(" "," ") , strip_tags(trim($_POST["name"])));
        $email = filter_var(trim($_POST["email"]), FILTER_SANITIZE_EMAIL);
        $phone = trim($_POST["phone"]);

		$address1 = trim($_POST["address1"]);
		$address2 = trim($_POST["address2"]);

		$bandname = trim($_POST["bandname"]);

		$urlfacebook = trim($_POST["urlfacebook"]);

		$urlinstagram = trim($_POST["urlinstagram"]);

		$datepicker = trim($_POST["datepicker"]);

        $row1 = trim($_POST["row1"]);
		$row2 = trim($_POST["row2"]);
		$row3 = trim($_POST["row3"]);
		$row4 = trim($_POST["row4"]);

		$bandmembers = trim($_POST["bandmembers"]);

		$concertpreference = trim($_POST["concertpreference"]);

		$gender = trim($_POST["gender"]);

		$venue = trim($_POST["venue"]);

		$comment1 = trim($_POST["comment1"]);

		$ideas = trim($_POST["ideas"]);

		$comment2 = trim($_POST["comment2"]);

		$lunch = trim($_POST["lunch"]);

		$additional_options = implode(' | ', $_POST["afternoontea"]);

		$footwear = trim($_POST["footwear"]);

		$option = trim($_POST["dropdown"]);

		$message = trim($_POST["message"]);

		$contact = trim($_POST["contact"]);

        if ( empty($name) OR !filter_var($email, FILTER_VALIDATE_EMAIL) OR empty($message)) {
            # Set a 400 (bad request) response code and exit.
            http_response_code(400);
            echo '<p class="alert alert-warning">Please complete the form and try again.</p>';
            exit;
        }

        $ip = $_SERVER['REMOTE_ADDR'];
        $response=file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=".$secretKey."&response=".$captcha."&remoteip=".$ip);
        $responseKeys = json_decode($response,true);

        if(intval($responseKeys["success"]) !== 1) {
          echo '<p class="alert alert-warning">Please check the the captcha form.</p>';
        } else {
            # Mail Content
            $content = "Name: $name\n";
            $content .= "Email: $email\n";
            $content .= "Phone: $phone\n";

			$content .= "Address 1: $address1\n";
			$content .= "Address 2: $address2\n";

			$content .= "Band Name: $bandname\n";

			$content .= "Facebook Profile: $urlfacebook\n";

			$content .= "Instagram Profile: $urlinstagram\n";

			$content .= "Preferred Date: $datepicker\n";

			$content .= "Snort Cocaine: $row1\n";
			$content .= "Smoke Marijuana: $row2\n";
			$content .= "Drink Bourbon: $row3\n";
			$content .= "Smash Guitars: $row4\n";

			$content .= "Band Members: $bandmembers\n";

			$content .= "Concert Preference: $concertpreference\n";

			$content .= "Gender: $gender\n";

			$content .= "Venue: $venue\n";

			$content .= "Venue Comments: $comment1\n";

			$content .= "Ideas: $ideas\n";

			$content .= "Ideas Comments: $comment2\n";

			$content .= "Lunch: $lunch\n";

			$content .= "Afternoon Tea: $additional_options\n";

			$content .= "Footwear: $footwear\n";

			$content .= "Option Selected: $option\n";

			$content .= "Preferred Contact: $contact\n";

			$content .= "Message: $message\n";

			
	
            # email headers.
            $headers = "From: $name <$email>";

            # Send the email.
            $success = mail($mail_to, $subject, $content, $headers);
            if ($success) {
                # Set a 200 (okay) response code.
                http_response_code(200);
                header('Location: https://www.mywebsitenz.com/test/thanks.html');
            } else {
                # Set a 500 (internal server error) response code.
                http_response_code(500);
                echo '<p class="alert alert-warning">Oops! Something went wrong, we couldnt send your message.</p>';
            }
        }

    } else {
        # Not a POST request, set a 403 (forbidden) response code.
        http_response_code(403);
        echo '<p class="alert alert-warning">There was a problem with your submission, please try again.</p>';
    }

?>

<pre><pre><!DOCTYPE html>
 
<html>
<head>
    <title>PHP MySQL Insert Tutorial</title>
        <script src='https://code.jquery.com/jquery-2.1.3.min.js'></script>
</head>
 
<body>
    <form action='insert.php' method='post' id='myform' >
    <p>
    <input type='text' name='username' placeholder='user name' id='username' />
    </p>
 
    <p>
    <input type='text' name='password' placeholder='password' id='password' />
    </p>
    
    <p>
    <input id = "minus" type="text" name="main" value="<?php echo $num1; ?>" autocomplete="off">
    </p>
            
    <button id='insert'>Insert</button>
    
    <p id='result'></p>
    </form>
</body>
</html>

<pre><?php
session_start();
if(isset($_POST['Submit']))
{
    $num_2    = "";
    $pass_num = $_POST['main'];
}
?>

<h2>minus</h2>
<br>
<form method="post" action="index.php">

<input type="text" name="state" value="<?php echo $num_2;?>" autocomplete="off">
<br>
<input type="hidden" name="rrr" value="<?php echo $pass_num;?>" autocomplete="off">
<br>
<br>
<input nam

$('#myform').submit(function(){
    return false;
});
 
$('#insert').click(function(){
    $.post(     
        $('#myform').attr('action'),
        $('#myform :input').serializeArray(),
        function(result){
            $('#result').php(result);
        }
    );
    });