Receiving syntax error with INSERT DATEDIFF - php

Question

I am receiving this error, and not being to insert DATEDIFF into the database. I can't seem to find the problem. I tried checking the syntax. Can anyone help please?
// Check connection
if (mysqli_connect_errno()) {
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
// escape variables for security
$pdate = mysqli_real_escape_string($con, $_POST['pdate']);
$mdate = mysqli_real_escape_string($con, $_POST['mdate']);
$amt = mysqli_real_escape_string($con, $_POST['amt']);
$first = mysqli_real_escape_string($con, $_POST['first']);
$last = mysqli_real_escape_string($con, $_POST['last']);
$pid = mysqli_real_escape_string($con, $_POST['pid']);
$cno = mysqli_real_escape_string($con, $_POST['cno']);
$madd = mysqli_real_escape_string($con, $_POST['madd']);
$bene = mysqli_real_escape_string($con, $_POST['bene']);
$swc = mysqli_real_escape_string($con, $_POST['swc']);
$bacc = mysqli_real_escape_string($con, $_POST['bacc']);
$bank = mysqli_real_escape_string($con, $_POST['bank']);
$badd = mysqli_real_escape_string($con, $_POST['badd']);
$bno = mysqli_real_escape_string($con, $_POST['bno']);
$sql="INSERT INTO contacts (
nodays, interest, pdate, mdate, amt, first, last,
pid, cno, madd, bene, swc, bacc, bank, badd, bno
)
VALUES (
DATEDIFF($mdate,$pdate) AS nodays,
(DATEDIFF($mdate,$pdate) * $amt / 365 * 0.1) AS interest,
'$pdate','$mdate','$amt','$first','$last',
'$pid','$cno','$madd','$bene','$swc','$bacc','$bank','$badd','$bno'
)";
if (!mysqli_query($con,$sql)) {
die('Error: ' . mysqli_error($con));
}
echo "Client record has been added to the database!";
mysqli_close($con);

Answer 1

The problem is that you try to execute 2 queries at the same time - only delimited by comma. That does not work. Seperagte them and execute them one by one.

Answer 2

You are inserting 2 more columns into contacts:
You say INSERT INTO CONTACTS with all the columns, but then you use
SELECT *, DATEDIFF... FROM CONTACTS. So you try to insert more columns into the same table.

Answer 3

I assume you are trying to calculate the nodays and interest as you do the insert, this should work:
INSERT INTO contacts (
nodays, interest, pdate, mdate, amt, first, last,
pid, cno, madd, bene, swc, bacc, bank, badd, bno
)
VALUES (
DATEDIFF($mdate,$pdate),
(DATEDIFF($mdate,$pdate) * $amt / 365 * 0.1),
'$pdate','$mdate','$amt','$first','$last',
'$pid','$cno','$madd','$bene','$swc','$bacc','$bank','$badd','$bno'
);