Error connecting to mySQL: SQLSTATE[HY093]: Invalid parameter number - php

Question

I've searched through multiple threads, but I don't see exactly where is the problem in my case.
It is showing the error on line 30: $findU->bindParam(':uid', $uid);. Not really sure if that's the real problem here.
Full code:
<?php
try {
$pdo = new \PDO('mysql:host=localhost;dbname=project;charset=utf8', 'root', '', [
PDO::ATTR_EMULATE_PREPARES => false,
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION
]);
if($_POST && isset($_POST['submit'])) {
$first = trim($_POST['first']);
$last = trim($_POST['last']);
$email = trim($_POST['email']);
$uid = trim($_POST['uid']);
$pwd = trim($_POST['pwd']);
//checks if there's an empty field
if(empty($first) || empty($last) || empty($email) || empty ($uid) || empty ($pwd)) {
header('Location: sign_up.php?signup=empty');
exit();
} else {
//checks inputs for invalid symbols through Regex
if(!preg_match("/^[a-zA-Z]/", $first) || !preg_match("/^[a-zA-Z]/", $last)) {
header('Location: sign_up.php?signup=invalid');
exit();
} else {
//checks if the email is in valid format
if(!filter_var($email, FILTER_VALIDATE_EMAIL)) {
header('Location: sign_up.php?signup=invalidemail');
exit();
} else {
//checks if the username is already in use
$findU = $pdo->prepare("SELECT * FROM `users` WHERE user_uid = ':uid'");
$findU->bindParam(':uid', $uid);
$result = $findU->execute();
if($result->fetchColumn() > 0) {
header("Location: sign_up.php?signup=usertaken");
exit();
} else {
//creating hash for password
$hashedPwd = password_hash($pwd, PASSWORD_DEFAULT);
//inserts new users into DB
$newUser = $pdo->prepare("INSERT INTO `users` (user_first, user_last, user_email,
user_uid, user_pwd) VALUES (':first', ':last', ':email', ':uid', ':hashedPwd')");
$newUser->execute(array(':first' => $first, ':last' => $last, ':email' => $email,
':uid' => $uid, ':hashedPwd' => $hashedPwd));
header("Location: sign_up.php?signup=success");
exit();
}
}
}
}
}
} catch(\PDOException $e) {
echo "Error connecting to mySQL: " . $e->getMessage();
echo "<code><pre>".print_r($e)."</pre></code>";
exit();
}
?>