PHP assign where in the link - php

Question

I have coded a php script that will generate a pdf file from some results. My select statement needs to be filtered with a where condition. Basically, I want to test my php script to see if it works well or not. How can I assign the where condition in the link itself?
this is my php code :
$query = "
SELECT
user_id
FROM TahdirUsers
WHERE
username = ':username'
";
$query_params = array(
':username' => $_POST['username']
);
try
{
$stmt = $db->prepare($query);
$result = $stmt->execute($query_params);
}
catch (PDOException $ex)
{
$response["success"] = 0;
$response["message"] = "Database Error1. Please Try Again!";
die(json_encode($response));
}
$row = $stmt->fetch();
if ($row)
{
if ($_POST['password'] === $row['password'])
{
$response["success"] = 1;
$response["message"] = "Login successful!";
die(json_encode($response));
}
else
{
$response["success"] = 0;
$response["message"] = "Invalid password!";
$pdf = new PDF(PDF_PAGE_ORIENTATION,PDF_UNIT,PDF_PAGE_FORMAT,true, 'UTF-8', true);
$pdf->SetFont('aefurat', '', 12, '', true);
// The first Parameter is localhost again unless you are retrieving data from a different server.
// The second parameter is your MySQL User ID.
// The third parameter is your password for MySQL. In many cases these would be the same as your OS ID and Password.
// The fourth parameter is the Database you'd like to run the report on.
$pdf->connect('xxxxxxxxx','xxxxxxx','xxxxxxxxx','xxxxxxxx');
// This is the title of the Report generated.
$attr=array('titleFontSize'=>24,'titleText'=>'THIS IS MY PDF FILE');
// This is your query. It should be a 'SELECT' query.
// Reports are run over 'SELECT' querires generally.
$pdf->mysql_report($query,false,$attr);
$pdf->Output('htmlout.pdf', 'I');
die(json_encode($response));
}
}
else
{
$response["success"] = 0;
$response["message"] = "this username in not in our database!";
die(json_encode($response));
}
To clarify my question. If my website link is www.testtesttest.com, how do I implement the where condition from the select statement in the URL

Answer 1

If my comment above is wrong, and your question is literally "how do I pass data from a URL to a PHP script", the answer is that you use the $_GET superglobal variable to access query strings in the standard format .../path/to/script.php?var1=value1&var2=value2...
So the URL http://amjad-test.site40.net/arabictest.php?username=imsop will execute 'arabictest.php' and populate the variable $_GET['username'] with the value 'imsop'. You then use that variable wherever you like in your code.