[RESOLVED] I need help = or == or ===

[Megahertza]

Here is my original code that lets any string pass and uses single "="

while($row = mysql_fetch_array($result)){
    If (($row[username] = $username) and ($row[password] = md5($password))) {
	echo "Thank you for logging in ".$username;
	}
}

If a use 3 "===" rather 1 equals then only the correct username and password works. Can some1 give me a small lesson in what situations a I should 1,2 or 3 equals.

[laserlight]

Use = for initialisation and assignment.
Use == for logical comparison.
Use === for logical and type comparison.