// Pointless commenting example:
// ----------
$username    = mysql_real_escape_string($_POST['username']); # Get username
$password    = mysql_real_escape_string($_POST['password']); # Get password
$age         = intval($_POST['age')];  #Get age
// ----------

// The above is useless because it's quite obvious of what's happening.

// Useful commenting:
// ----------
# Get information from POST

$username    = mysql_real_escape_string($_POST['username']);
$password    = mysql_real_escape_string($_POST['password']);
$age         = intval($_POST['age')];
// ----------

// This way saves time.

// Also, a good place to put comments, is when you're making a new function or class.
// Describe the inputs and outputs a bit.
// ----------

/*
    Function: salt_crypt
    Desc: Encrypts a string using a salt.
    Inputs:
        $string (string): The string to be encrypted
        $salt (string)  : The salt to use.
*/

function salt_crypt($string, $salt)
{
    $return = md5(md5($string) . $salt);
    return($return);
}

// ----------

/*

Why comment like that at the beginning of a function? It will save a lot of time. Saves the person reading from having to go through the code for that function.

*/