Add 'username' to URL from 'user_id' with PHP for Dummies - php

Question

I am attempting to publish a question which I know has many answers floating around Stack Overflow. However I, for some reason, just cannot seem to get the information in them to click. I will post my sources at the end of the question.
The question being, how can I pull a user_id from a URL & mod_rewrite it show the username.
Instead of this:
domain/user/index.php?user_id=1
I get this:
domain/username
I had customized other SO answers to my needs on my .htaccess file to this:
<IfModule mod_rewrite.c>
Options +FollowSymlinks
RewriteEngine On
RewriteOptions MaxRedirects=1
RewriteCond %{REQUEST_FILENAME} -f [NC,OR]
RewriteCond %{REQUEST_FILENAME} -d [NC]
RewriteRule .* - [L]
RewriteRule ^user/([^/]+)$ index.php?user_id=$1
</IfModule>
The if statement at the top of my user page looks like this:
if (isset($_GET['user_id']) || isset($_GET['username']) && queryUserId($user_id)) {
// Render Page Content
} else {
header('Location: sign_up.php');
}
And my queryUsername functions looks like this:
function queryUsername($username) {
$conn = dbConnect('read');
$sql = "SELECT * FROM users WHERE user_id = '".$username."'";
$result = $conn->query($sql) or die(mysqli_error($conn));
$row = $result->fetch_assoc();
return $row['username'];
$username = $row['username'];
}
I have successfully implemented unique URL's for users, and am accessing Account Profiles just fine when appending different user_id's to my URL, so what am I missing to get the username written to the URL instead of variable strings and folder structure?
Cheers!
Other SO Questions:
Directly adding username to URL PHP
Get username from URL in PHP
Using mod rewrite to change URL with username variable
AddedBytes Article
URL Rewriting for Beginners

Answer 1

This line is wrong according to your information:
RewriteRule ^user/([^/]+)$ index.php?user_id=$1
It should at least be:
RewriteRule ^user/([^/]+)$ /user/index.php?user_id=$1 // for a url like domain/user/username
or using your description:
RewriteRule ^([^/]+)$ /user/index.php?user_id=$1 // for a url like domain/username
Also, you are not setting any variable with the name of username in the code you have shown, so the check for $_GET['username'] is unnecessary.
Your check in php should look something like:
if ( isset($_GET['user_id']) && queryUserId($_GET['user_id']) ) {
Apart from that you should not use the deprecated mysql_* functions and use prepared statements as you have an sql injection problem now.
Also note that using two return statements after each other only returns the first value.
Edit: There seems to be some confusion between the user ID and the username. If the value in the url is a username, you'd better call it username in both the .htaccess file and php to avoid confusion with the user ID:
RewriteRule ^([^/]+)$ /user/index.php?username=$1 // for a url like domain/username}
and
if ( isset($_GET['username']) && queryUserName($_GET['username']) ) {
in the function (using the deprecated functions just to illustrate...):
function queryUserName($username) {
$conn = dbConnect('read');
$sql = "SELECT * FROM users WHERE username = '".$username."'";
...
}