include php date variable in MySQL SELECT query - php

Question

I have a main php page where the user inputs a date range to view a table. I am using a jquery datepicker and am passing the values from the datepicker to another php page via ajax where my SELECT statement occurs. The values pass successfully and I can echo them back to the original page in the "yyyy-mm-dd" format (which is what I want). Then, I need to run a SELECT statement pulling only rows out where the orderdate is between the datepicker values picked by the user. When I run my query without the WHERE clause, the query is successful, and my table displays data. When I had the Where clause, the table headers displayed, but the data wouldn't. How do I get the variables to work with the query without the risk of SQLi?
Here is my code pulling datepicker values from ajax (and an attempt to convert the string to a date format - I am new to this and wasn't sure if that was the problem):
//Get date range.
$revenuefromajax=$_POST['revenuefromtext'];
$revenuetoajax=$_POST['revenuetotext'];
$revenuefromstring = strtotime($revenuefromajax);
$revenuetostring = strtotime($revenuetoajax);
$revenuefrom=date("Y-m-d", $revenuefromstring);
$revenueto=date("Y-m-d", $revenuetostring);
echo $revenuefrom;
echo $revenueto;
MySQL Query:
$sql = "SELECT invoices.id, invoices.orderdate, invoices.stagestatus, FORMAT(TRIM(LEADING '$' FROM invoices.totalprice), 2) AS totalprice, clients.company, lineitems.invoiceid, FORMAT((lineitems.width * lineitems.height) /144, 2 ) AS sqft, lineitems.quantity AS qty, FORMAT((invoices.totalprice / ((lineitems.width * lineitems.height) /144)), 2) as avgsqftrevenue, FORMAT((TRIM(LEADING '$' FROM invoices.totalprice) / lineitems.quantity), 2) AS avgunitrevenue
FROM clients
INNER JOIN invoices ON clients.id = invoices.clientid
INNER JOIN lineitems ON invoices.id = lineitems.invoiceid
WHERE invoices.ordedate BETWEEN ('".$revenuefrom."') AND ('".$revenueto."')
ORDER BY invoices.id DESC
LIMIT 10";
$result = $conn->query($sql);
echo "<table id='revenueReportA' align='center' class='report_DT'>
<tr>
<th>Customer</th>
<th>SG</th>
<th>Revenue</th>
<th>SQ FT</th>
<th>AVG Revenue Per SQ FT</th>
<th>Number of Units</th>
<th>AVG Revenue Per Unit</th>
</tr>";
if ($result = $conn->query($sql)) {
// fetch associative array
while ($row = $result->fetch_assoc()) {
echo "<tr>";
echo "<td>" . $row['company'] . "</td>";
echo "<td>" . $row['id'] . "</td>";
echo "<td>" ."$". $row['totalprice'] . "</td>";
echo "<td>" . $row['sqft'] ." ". "ft<sup>2</sup>". "</td>";
echo "<td>" ."$". $row['avgsqftrevenue'] . "</td>";
echo "<td>" . $row['qty'] . "</td>";
echo "<td>" ."$". $row['avgunitrevenue'] . "</td>";
echo "</tr>";
}
echo "</table>";