PHP MYSQL verify user from table and set SESSION 'username' - php

Question

I have a forum page and want a simple login for user with usernames from a predefined mysql user table. I use a login.php form file link from the forum, a get $_POST username, then use a login_handle.php file that calls a function to connect to the DB, query the users array, and try to validate that the $_POST username is in the queried list array.
The function is below, and then the call in login_handle.php I'm getting various errors and don't know if this is at all a good approach. I also want to start a session during the form and verification that can grab the $_POST username as a $_SESSION username and apply to subsequent forum pages.
function isUsername($username){ //Test if proper Username from array.
$query = "SELECT username FROM users";
$result = mysql_query($query);
$usernames = mysql_fetch_assoc($result);
$isUname = true;
if(!in_array("username", $usernames)) {
echo "Please enter valid user name.<br>";
$isUname = false;
} //Search for proper username in username array.
return $isUname;
}
------------------handler call-----------
$username = $_POST["username"];
$password = $_POST["password"];
if(isUsername($username)==true){ // Check if username is valid.
//$Uname = $_SESSION['username'];
//echo "Username = " . $Uname;
echo 'go to forum';
}

Answer 1

First, mysql is deprecated. Please use mysqli.
Second, why don't you use something like...
function isUsername($username){
$query = "SELECT username FROM users WHERE username == '" . $username . "'";
Third: did you search and research?
These kind of question can be easily find everywhere.

Answer 2

As simple as it is , you need to query the specific username from $_POST , not the whole usertable.
I think requesting the number of rows ( number of apparition is a good way to get if user is in database or not , you can make it greater (>=) instead of one user condition (==)).
function isUsername($username){ //Test if proper Username from array.
$query = "SELECT username FROM users where username='$username'";
$result = mysql_query($query);
$rows = mysql_num_rows($result);
$isUname = true;
if($rows==1) {
$isUname = true;
}else{
echo "Please enter valid user name.<br>";
$isUname = false;
}
return $isUname;
}
I used nearly the same function when I manually assigned a txt array to a variable $username to compare. Now that I am using a user table I merely want to assign the an array of the queried users (thought mysql_fetch_assoc($result) creates the same type of assoc. array) to $username instead of the hard copied elements where it worked with before. Is the array produced with the query different than the $usernames=array("jes34","pes22","jis44","jll124"); that prevents me from doing this?
function isUsername($username){ //Test if proper Username from array.
$usernames=array("jes34","pes22","jis44","jll124");
$isUname = true;
if(!in_array($_POST["username"], $usernames)) {
echo "Please enter valid user name.<br>";
$isUname = false;
} //Search for proper username in username array.
return $isUname;
}
-----function call---
if(isUsername($username)==true){ do something }