Hàm array_keys() trong PHP - PHP Function

$array = array(
	0 => "php",
	"js" => "javascript"
);
echo "<pre>";
	print_r(array_keys($array));
echo "</pre>";

$array = array(
	"php",
	"js",
	"css",
	"php",
	"html",
	"php"
);
echo "<pre>";
	print_r(array_keys($array));
echo "</pre>";

$array = array(
	"language" => array(
					"php",
					"css",
					"html",
					"python"
				),
    "framework"  => array(
			    	"laravel",
			    	"boostrap",
			    	"nodejs"
			    )
);
echo "<pre>";
	print_r(array_keys($array));
echo "</pre>";

Array
(
    [0] => 0
    [1] => js
)
Array
(
    [0] => 0
    [1] => 1
    [2] => 2
    [3] => 3
    [4] => 4
    [5] => 5
)
Array
(
    [0] => language
    [1] => framework
)