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<?php

$array = array(0 => 100, "color" => "red");

print_r(array_keys($array));

$array = array("blue", "red", "green", "blue", "blue");

print_r(array_keys($array, "blue"));

$array = array("color" => array("blue", "red", "green"),

               "size"  => array("small", "medium", "large"));

print_r(array_keys($array));

?>

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Array

(

    [0] => 0

    [1] => color

)

Array

(

    [0] => 0

    [1] => 3

    [2] => 4

)

Array

(

    [0] => color

    [1] => size

)