PDO Query Check for Duplicate Username Upon Submission

ERROR: Parse error: syntax error, unexpected T_ENCAPSED_AND_WHITESPACE, expecting T_STRING or T_VARIABLE or T_NUM_STRING on line 7

Trying to use PDO to make this connection and form the query that checks for a username if it exists or not upon input into field after submit is pressed.

HTML:

<form action="inc/check_regUsr.php" method="post" id="userLogon">
    <div class="field required">
        Username: <input type="text" name="regduser" tabindex="1" /><br />
        </div>
        <div class="field required">
        Password: <input type="password" name="re开发者_StackOverflow中文版gdpass" tabindex="2" /><br />
        </div>
        <input type="submit" name="submitUser" />
</form>

PHP

<?php
#Login Details
require_once('dbcred.php');
$conn = new PDO("mysql:host=$host;dbname=$db", $user, $pass);

#Check for Existing User
$q = $conn->query("SELECT uname FROM Student WHERE $_POST['regduser'] = uname");
$stmt = $conn->prepare($q);
$r->execute($q);
if($q($r)>= 1){ #if there are 1 or more users with enter username, deny.
echo "Sorry, username already exists";
}
else{
echo "Success";
}

?>

Enclose your complex variables in {} inside a double-quoted string:

$q = $conn->query("SELECT uname FROM Student WHERE {$_POST['regduser']} = uname");
// -----------------------------------------------^^^^^^^^^^^^^^^^^^^^^

It looks like your SQL WHERE clause is backward though, and missing quotes. Should be

WHERE uname = '{$_POST['regduser']}'

You have another problem, where you are first calling $conn->query() and then attempting to create a prepared statement.

The call to query() is unnecessary and actually dangerous. Instead create a proper prepared statement with ? placeholders:

$stmt = $conn->prepare("SELECT uname FROM Student WHERE uname = ?");
$stmt->bindParam(1, $_POST['regduser'], PDO::PARAM_STR);    
$stmt->execute();

Since you're already using PDO, you might as well take advantage of the parameters feature which provides great protection against SQL injection attacks.

$conn = new PDO("mysql:host=$host;dbname=$db", $user, $pass);

$stmt = $conn->prepare("SELECT uname FROM Student WHERE ? = uname");
$params = array($_POST['regduser']);
$stmt->execute($params);
if ($stmt->rowCount() > 0) {

echo "Sorry, username already exists";
}
else{
echo "Success";
}