Tracking Issue im not sure how to fix

Basically what im wanting to do is select a problem from the table on the database and echo it. I have the following code;

<?php 
$UserID = `UserIDProblem`; 
if(isset($_POST['track'])) 
{ 
$username = "student"; 
$password = "student"; 
$hostname = "localhost";  
$db       = "details"; 
$link = new mysqli($hostname, $username, $password, $db); 
if(! $link ) 
{ 
  die('Could not connect: ' . mysql_error()); 
} 
if(isset($_POST[`UserIDProblem`])){ $UserID = $_POST[`UserIDProblem`]; } 
$USERID = $_POST['UserIDProblem']; 
$sqli = "SELECT * FROM problem WHERE UserIDProblem = '$UserID'"; 
$UserID = mysqli_real_escape_string($link, $_POST['UserIDProblem']); 
$retval = mysqli_query( $link,$sqli  ); 
if(! $retval ) 
{ 
  die('Could not find data: ' . mysqli_error($link)); 
} 
echo "Data found successfully\n"; 
echo $sqli; 
mysqli_close($link); 
} 
else 
{ 
?>

But its not working. Am I doing the statement wrong? Basically if the USER ID is say for example 3, I want it to echo the results for USER ID being 3 but its not working. Any help? I'm not sure how to fix this. Thanks.