Php sort header SQL query

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PHP

MySQL

I have the below code, but mysql query keeps given an error, I am trying to use the header column to sort ascending/descending but doesn't work.

Gives the following message:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1


any help would be great.

What I have tried:

<pre lang="PHP">
<pre>function process() {
	global $DatabaseServer, $DatabaseUser, $DatabasePswd, $DatabaseName, $AgentPin;
	$cn=mysqli_connect($DatabaseServer, $DatabaseUser, $DatabasePswd, $DatabaseName);
	$sql = "SELECT pin.username, pendingindex.* FROM pin join sorttest ON pin.AgentID = sorttest.AgentID join pendingindex ON sorttest.pendreckey = pendingindex.pendreckey ORDER BY" . $sort . " " . $sort_order;


// sets id as default sort (you can set something else than id)
$sort = (isset($_GET['sort'])) ? $_GET['sort'] : 'pendreckey';

// define sort order
$sort_order = 'asc';
if(isset($_GET['sort_by'])) {
 if($_GET['sort_by'] == 'asc') {
  $sort_order = 'desc';
 } else {
  $sort_order = 'asc';
 }
}

// query table
$query  = mysqli_query($cn,$sql) or die(mysqli_error($cn));
$keys   = mysqli_fetch_array($query, MYSQLI_ASSOC);

// html table
echo "<table border='1'>";

// print out table columns
echo "<tr>";
foreach(array_keys($keys) as $key) {
 switch($sort) {
  case $key :
   $order_by = $key;
   break;
 }

 if($sort==$key) {
  echo "<td bgcolor=white><a href='?sort=$key&sort_by=" . $sort_order . "'>$key</a></td>";
 } else {
  echo "<td bgcolor=white><a href='?sort=$key&sort_by=" . $sort_order . "'>$key</a></td>";
 }
}

echo "</tr>";

// print out table contents
$query  = mysqli_query($cn,$sql);
while($row = mysqli_fetch_array($query, MYSQLI_ASSOC)) {
 echo "<tr>";
 foreach($row as $rows) {
  echo "<td>" . $rows . "</td>";
 }
 echo "</tr>";
}

echo "</table>";
  
echo '<br><br><br>';
    }