Update MySql Query for user profile update using php - php

Question

I am trying to update user profile by overwriting current data in the MySql.
It isn't working properly its stating it echos my "the name doesn't exist"
Here is my php code:
``````
<?php
// see if the form has been completed
session_start();
include_once("php_includes/check_login_status.php");
include_once("php_includes/db_conx.php");
// Initialize any variables that the page might echo
$u = "";
$firstname = "";
$surname = "";
$gender = "Male";
$country = "";
$weight = "";
$height = "";
$password = "";
$password2 = "";
if(isset($_GET["u"])){
$u = preg_replace('#[^a-z0-9]#i', '', $_GET['u']);
}
$sql = "SELECT * FROM users WHERE username='$u' AND activated='1' LIMIT 1";
$user_query = mysqli_query($db_conx, $sql);
// check if the user exists in the database
while ($row = mysqli_fetch_array($user_query, MYSQLI_ASSOC)) {
$firstname = $row["firstname"];
$surname = $row["surname"];
$weight = $row["weight"];
$height = $row["height"];
$email = $row["email"];
$gender = $row ["gender"];
}
if (isset($_POST['submit'])){
$firstname = $_POST['firstname'];
$surname = $_POST['surname'];
$weight = $_POST['weight'];
$height = $_POST['height'];
$email = $_POST['email'];
$gender = $_POST['gender'];
mysql_connect ("localhost","root","pass123"); mysql_select_db('worldoi5_social');
// check if that user exist
$exists = mysql_query ("SELECT * FROM users WHERE username='$u' ") or die ("query cant connect");
if (mysql_num_rows ($exists) != 0) {
// update the description in the database
mysql_query("UPDATE users SET firstname='$firstname', surname='$surname', weight='$weight', height='$height' WHERE username='$u'") or die ("update could not be applied");
echo "successful";
} else echo "the name does not exist";
}
?>
I have made the code work with less code, but since i have added more it stopped working, and the data reads fine, I do not see the errors or mistake.
here is a bit of the HTML aswell:
<form action="user1.php" method="POST">
<div>
<p>First Name: <input type="text" name="firstname" id="firstname" value="<?=$firstname?>"></p>
<p>Surname: <input type="text" name="surname" id="surname" value="<?=$surname?>"></p>
<p>Weight: <input type="text" name="weight" id="weight" value="<?=$weight?>"></p>
<p>Height: <input type="text" name="height" id="height" value="<?=$height?>"></p>
<p> <input type="submit" name="submit" id="submit" value="Update Description"></p>
</div>
</form>
</body>

Answer 1

I figured out the answer:
from the php code line
$exists = mysql_query ("SELECT * FROM users WHERE username='$u' ")
the single and double quots where messed up so i had to use it like this;
$exists = mysql_query ("SELECT * FROM users WHERE firstname='" . $firstname . "'")
then it read it properly also i decided to use firstname as the selective