username password 
test 123456

 mysql_fetch_assoc()
 array([username]=>'test',[password]=>'123456')

mysql_fetch_array()
array([0]=>'test',[1]=>'123456')

array ( 0 => 1, 1 => 'a', 2 => 'hello', )

array(3) { [0]=> int(1) [1]=> string(1) "a" [2]=> string(5) "hello" }