Arrays asociativos

<?php

$m=[[1,2,3],
    [4,5,6,7,8],
    [7,8,9]];
print_r($m);
echo $m[1][1];  //Imprime 5 (fila 1, columna 1)
echo $m[1][3]; //Imprime 7 (fila 1, columna 3)
echo "<hr>";
//recorrer array
foreach($m as $c1=>$fila){
    foreach($fila as $c2=>$v){
        echo "($c1,$c2) $v ";
    }
    echo "<br>";
}
echo "<hr>";

$ma=[["nombre"=>"Ana","mail"=>"p@p.com","ciudad"=>"BCN"],
    ["nombre"=>"Eva","mail"=>"e@p.com","ciudad"=>"BCN"],
    ["nombre"=>"Rosa","mail"=>"r@p.com","ciudad"=>"BCN"],
    ];
print_r($ma);
echo $ma[1]['mail']; //El mail de Eva (fila 1, columna 'mail'
echo "<hr>";
//recorrer array
foreach($ma as $c1=>$fila){
    foreach($fila as $c2=>$v){
        echo "($c1,$c2) $v ";
    }
    echo "<br>";
}
echo "<hr>";
$aa=["gerencia"=>["nombre"=>"Ana","mail"=>"p@p.com","ciudad"=>"BCN"],
    "administracion"=>["nombre"=>"Eva","mail"=>"e@p.com","ciudad"=>"BCN"],
    "fábrica"=>["nombre"=>"Rosa","mail"=>"r@p.com","ciudad"=>"BCN"],
    ];
print_r($aa);
echo $aa['administracion']['mail']; //El mail de Eva (fila 'administracion', columna 'mail'
echo "<hr>";
//recorrer array
foreach($aa as $c1=>$fila){
    foreach($fila as $c2=>$v){
        echo "($c1,$c2) $v ";
    }
    echo "<br>";
}
echo "<hr>";

$a = [];
$n = 4;
$cont = 1;
for ($i = 0; $i < $n; $i++) {
    for ($j = 0; $j < $n; $j++) {
        $a[$i][$j] = $cont;
        $cont++;
    }
}

print_r($a);

echo $a[2][1]; //¿Qué imprime esto?

$usuarios = [["Nombre" => "Eva", "Aficiones" => ["Musica", "Cine"]],
    ["Nombre" => "Ana", "Aficiones" => ["Baile", "Cine"]],
    ["Nombre" => "Rosa", "Aficiones" =>
        ["Deportivas" => ["Fútbol", "Tenis"],
            "Culturales" => ["Música", "Teatro"]]],
];

echo $usuarios[1]["Aficiones"][0]; //Baile
echo $usuarios[2]["Aficiones"]["Culturales"][0]; //Música