How to get the values in a database table to an array using php code?
$query = "...";
$sql = mysql_query($query);
while($row = mysql_fetch_assoc($sql)) {
    $user_id = $row['user_id'];
    // ...
}

$username = mysql_real_escape_string($_POST['username']); //escaping the string, will use it in a query
$password = $_POST['password'];

$result = mysql_query("SELECT password FROM users WHERE username = '$username'");

if (mysql_num_rows($result) == 1) { // Expecting one result
     $userdata = mysql_fetch_array($result);
     if ($password == $userdata['password']) {
        // USER IS AUTHORISED
     } else {
        // USER IS NOT AUTHORISED
    }
}

$username="username";
    $password="password";
    //Specify MySQL database to connect to
    $database="database";
    //Create connection and select the appropriate database
    mysql_connect("localhost",$username,$password);
    @mysql_select_db($database) or die( "Unable to select database");

    //Get the username and password from posted form
    $username = mysql_real_escape_string($_POST['username']);
    $password = mysql_real_escape_string($_POST['password']);

    //Build the query to look for users that exist with that username/password combination
    //Here I'm using the users table within my database
    $query = "SELECT * FROM users where username="$username" and password = "$password" ";
    //Retrieve the results of the query, and also aquire the number of records returned
    $result = mysql_query($query);
    $num = mysql_numrows($result);

    //Check to see how many records are returned
    if ($num>0)
    {
        //User login was successful

        echo 'success login';
    } else {
        //Login was unsuccessful
       echo 'user name not found';
    }

<form method="post">
<input type="text" name="username" />
<input type="password" name="password" />
<input type="submit" name="login"  value="login"/>
</form>

if(isset($_POST['login']))
{
  $username=$_POST['username'];
  $password=$_POST['password'];

  $username=mysql_real_escape_string($username);
  $password=mysql_real_escape_string($password);
  $query = "select username,password from tableName
            where username='$username' and password='$password'";
$sql = mysql_query($query);

 if(mysql_num_rows($sql) == 1)
 {
   // Login success save username in session redirect to homepage
     header("location:homepage.php");
     exit();
 }
 else
 {
   // Display error message
 }
}