SQL insert into DB using PHP - issue with INSERT INTO - php

Question

I'm trying to convert a previous line I had where I was calling something back from the database, and insert it instead.
This is the function I have, but I can't get the INSERT INTO to work correctly
I've already debugged that: the connection to the DB is working fine, the session var for user is set and that the $avatarID is present.
if(!empty($_SESSION['user'])){
$avatarID = $_POST['avatarID'];
$avatarID = mysql_real_escape_string(trim($_POST['avatarID']));
// Insert into DB
$sql = "INSERT INTO `users` (`avatar`) VALUES ('{$avatarID}') WHERE `username` = '".$_SESSION['user']."'";
$query = mysql_query($sql);
if($query === false){
return false;
}else{
return true;
}
header('Location: profile.php');
}
I think it's an issue with the $sql line. I'm not getting any errors other than a simple blankpage/dead screen.
Attempted changing to just the following:
// Insert into DB
$query = mysql_query("INSERT INTO `user` (`avatar`) VALUES ('{$avatarID}'") or die(mysql_error());
Edit OK so I realise the mistake I have made, as this should be an UPDATE WHERE not INSERT INTO. But I am still struggling to get the query details correct even when using UPDATE and WHERE. But still no result:
<?php session_start();
require 'connect.php';
if(!empty($_SESSION['user'])){
$avatarID = $_POST['avatarID'];
$avatarID = mysql_real_escape_string(trim($_POST['avatarID']));
// Insert into DB
$sql = "UPDATE `users` SET `avatar`='{$avatarID}' WHERE `username` = '".$_SESSION['user']."'";
$query = mysql_query($sql);
if($query === false){
return false;
}else{
return true;
}
header('Location: profile.php');
}else{
header('Location: choose-avatar.php');
}
?>

Answer 1

Use UPDATE instead of INSERT
$sql = "UPDATE `users` SET `avatar`='{$avatarID}' WHERE `username` = '".$_SESSION['user']."'";