My PHP program doesn't run

FIRST FILE: index.php

<!DOCTYPE html>
<html>
<head>
<title>
Test program
</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body>

<a href="http://www.nippondream.com\testitems\01a.mp3">Play the tape</a>

<form action="insert.php" method="POST">
Please choose the right answer!!!!

<table style="width: 20%">
<tr>
<th>Right <input type="radio" name="a" value="a" required/> </th>
<th>Light<input type="radio" name ="b" value="b" required/> </th>
<th>Can't tell<input type="radio" name ="c" value="c" required/> </th>
</tr>
</table>
<input type="submit" value="Enter"/>
</form>

</body>
</html>

 

********************

SECOND FILE: insert.php

<html>
<body>

<?php

$conn = mysqli_connect("localhost","root","","quizbank");
mysqli_query($conn, "SET NAMES 'UTF8'") or die("ERROR: ". mysqli_error($conn));

$sql="INSERT INTO users (user_id,item_id,response)
VALUES
('$_POST[a]','$_POST[b]','$_POST[c]') ";

if (!mysql_query($sql,$conn))
{
die('Error: ' . mysql_error());
}
echo "1 record added";
mysql_close($conn)

?>

</body>
</html>

cable two male XLR and one female

https://www.amazon.com/Yeung-Qee-Female-Splitter-Y-Adapter/dp/B07BCCGHG7/ref=asc_df_B07BCCGHG7/?tag=hyprod-20&linkCode=df0&hvadid=312172947812&hvpos=1o3&hvnetw=g&hvrand=15066752416649195112&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9007576&hvtargid=pla-571935483075&psc=1

 

https://www.amazon.com/dp/B000Y021NA/ref=psdc_172544_t1_B00CODJFIE

 

https://www.sweetwater.com/store/detail/PWP047Z--daddario-planet-waves-pw-p047z-xlr-male-to-1-4-inch-female-balanced-adapter?mrkgcl=28&mrkgadid=3336039928&rkg_id=0&product_id=PWP047Z&campaigntype=shopping&campaign=aaShopping%20-%20SKU%20-%20Accessories&adgroup=Cables%20-%20D%27Addario%20Planet%20Waves%20-%20pwp047z&placement=google&adpos=1o1&creative=342727179485&device=c&matchtype=&network=g&gclid=Cj0KCQjw7sDlBRC9ARIsAD-pDFpF-qGYRYwTLf3fovTpsh7OFYIWWl85iEr73ecIeMLDT8PtEqwHF1saAn6PEALw_wcB