How to use passed variable in a PDO prepared SELECT WHERE statement - php

Question

I am trying to figure out how to use a php variable in a PDO prepared statement. The code below returns nothing. The part that I'm not sure about is
$stmt = $conn->prepare("SELECT * FROM laptop WHERE '$ckey' = :avalue");
I'm trying to use a variable for the key and the value, It doesn't seem to be returning results. The switch statement is meant to look for which field the user filled out. This is a basic search for a rudimentary inventory system. I know there are open source solutions available, but my company won't let us use open source software on the network. I also realize the output isn't formatted into a table, but I figured I'd get the query to work, then worry about prettying up the output. I also realize there are some similar questions, but I haven't seen anyone trying to use variables in the WHERE part of a statement. If it's because it won't work, and everyone knows this but me, I apologize.
Here is the entire code block.
function mkquery($ckey,$cdata)
{
try {
$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
// set the PDO error mode to exception
}
catch(PDOException $e)
{
echo $sql . "<br>" . $e->getMessage();
}
$stmt = $conn->prepare("SELECT * FROM laptop WHERE '$ckey' = :avalue");
//$stmt is the prepared query, you execute it to perform the query
$stmt->bindParam(':avalue',$ckey, PDO::PARAM_STR);
$stmt->execute();
$result = $stmt->fetchAll();
foreach($result as $val ) {
echo $val['Model'] . " ";
echo $val['Name'] . " ";
echo $val['DellTag'] . " ";
echo $val['HHSCTag'] . " ";
echo $val['OS'] . " ";
echo $val['KBOX'] . " ";
echo $val['SB'] . " ";
echo $val['Issued'] . " ";
echo $val['Tech'] . " ";
echo "<br>";
}
}
switch (isset($_POST)) {
case isset($_POST['name']):
$selector = 'name';
mkquery($selector,$_POST['name']);
break;
case isset($_POST['model']):
$selector = 'model';
mkquery($selector,$_POST['model']);
break;
case isset($_POST['delltag']):
$selector = 'delltag';
mkquery($selector,$_POST['delltag']);
break;
case isset($_POST['hhsctag']):
$selector = 'hhsctag';
mkquery($selector,$_POST['hhsctag']);
break;
case isset($_POST['city']):
$selector = 'city';
mkquery($selector,$_POST['city']);
break;
case isset($_POST['kbox']):
$selector = 'kbox';
mkquery($selector,$_POST['kbox']);
break;
case isset($_POST['sb']):
$selector = 'sb';
mkquery($selector,$_POST['sb']);
break;
case isset($_POST['issued']):
$selector = 'issued';
mkquery($selector,$_POST['issued']);
break;
case isset($_POST['tech']):
$selector = 'tech';
mkquery($selector,$_POST['tech']);
break;
default:
echo 'No input on form detected';
break;
}
a
$conn = null;