Arreglos asociativos

$notas['luis'] = 9.5;
$notas = Array
(
    [luis] => 9.5
)

$notas['ana'] = 10.0;
$notas = Array
(
    [luis] => 9.5
    [ana] => 10
)

$notas['luis'] = 6.5;
$notas = Array
(
    [luis] => 6.5
    [ana] => 10
)

$notas[] = 'ana';
$notas = Array
(
    [luis] => 6.5
    [ana] => 10
    [0] => ana
)

$notas[3] = 'diana';
$notas = Array
(
    [luis] => 6.5
    [ana] => 10
    [0] => ana
    [3] => diana
)

$notas[] = 'fiona';
$notas = Array
(
    [luis] => 6.5
    [ana] => 10
    [0] => ana
    [3] => diana
    [4] => fiona
)