php-isset和empty

<?php

$a = null;

if(isset($a)){
    echo "is set"."\n";
}
else{
    echo "not set"."\n";
}

if(empty($a)){
    echo "is empty"."\n";
}
else{
    echo "not empty"."\n";
}

?>

not set
is empty

<?php

$a = 0;

if(isset($a)){
    echo "is set"."\n";
}
else{
    echo "not set"."\n";
}

if(empty($a)){
    echo "is empty"."\n";
}
else{
    echo "not empty"."\n";
}

?>

is set
is empty